Fix 'hash_table::expand' to destruct stale Value objects

Thus plugging potentional memory leaks if these have non-trivial
constructor/destructor.

See
<https://stackoverflow.com/questions/6730403/how-to-delete-object-constructed-via-placement-new-operator>
and others.

As one example, compilation of 'g++.dg/warn/Wmismatched-tags.C' per
'valgrind --leak-check=full' improves as follows:

     [...]
    -
    -104 bytes in 1 blocks are definitely lost in loss record 399 of 519
    -   at 0x483DFAF: realloc (vg_replace_malloc.c:836)
    -   by 0x223B62C: xrealloc (xmalloc.c:179)
    -   by 0xA8D848: void va_heap::reserve<class_decl_loc_t::class_key_loc_t>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int, bool) (vec.h:290)
    -   by 0xA8B373: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_ptr>::reserve(unsigned int, bool) (vec.h:1858)
    -   by 0xA8B277: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_ptr>::safe_push(class_decl_loc_t::class_key_loc_t const&) (vec.h:1967)
    -   by 0xA57481: class_decl_loc_t::add_or_diag_mismatched_tag(tree_node*, tag_types, bool, bool) (parser.c:32967)
    -   by 0xA573E1: class_decl_loc_t::add(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32941)
    -   by 0xA56C52: cp_parser_check_class_key(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32819)
    -   by 0xA3AD12: cp_parser_elaborated_type_specifier(cp_parser*, bool, bool) (parser.c:20227)
    -   by 0xA37EF2: cp_parser_type_specifier(cp_parser*, int, cp_decl_specifier_seq*, bool, int*, bool*) (parser.c:18942)
    -   by 0xA31CDD: cp_parser_decl_specifier_seq(cp_parser*, int, cp_decl_specifier_seq*, int*) (parser.c:15517)
    -   by 0xA43C71: cp_parser_parameter_declaration(cp_parser*, int, bool, bool*) (parser.c:24242)
    -
    -168 bytes in 3 blocks are definitely lost in loss record 422 of 519
    -   at 0x483DFAF: realloc (vg_replace_malloc.c:836)
    -   by 0x223B62C: xrealloc (xmalloc.c:179)
    -   by 0xA8D848: void va_heap::reserve<class_decl_loc_t::class_key_loc_t>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int, bool) (vec.h:290)
    -   by 0xA8B373: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_ptr>::reserve(unsigned int, bool) (vec.h:1858)
    -   by 0xA8B277: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_ptr>::safe_push(class_decl_loc_t::class_key_loc_t const&) (vec.h:1967)
    -   by 0xA57481: class_decl_loc_t::add_or_diag_mismatched_tag(tree_node*, tag_types, bool, bool) (parser.c:32967)
    -   by 0xA573E1: class_decl_loc_t::add(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32941)
    -   by 0xA56C52: cp_parser_check_class_key(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32819)
    -   by 0xA3AD12: cp_parser_elaborated_type_specifier(cp_parser*, bool, bool) (parser.c:20227)
    -   by 0xA37EF2: cp_parser_type_specifier(cp_parser*, int, cp_decl_specifier_seq*, bool, int*, bool*) (parser.c:18942)
    -   by 0xA31CDD: cp_parser_decl_specifier_seq(cp_parser*, int, cp_decl_specifier_seq*, int*) (parser.c:15517)
    -   by 0xA53385: cp_parser_single_declaration(cp_parser*, vec<deferred_access_check, va_gc, vl_embed>*, bool, bool, bool*) (parser.c:31072)
    -
    -488 bytes in 7 blocks are definitely lost in loss record 449 of 519
    -   at 0x483DFAF: realloc (vg_replace_malloc.c:836)
    -   by 0x223B62C: xrealloc (xmalloc.c:179)
    -   by 0xA8D848: void va_heap::reserve<class_decl_loc_t::class_key_loc_t>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int, bool) (vec.h:290)
    -   by 0xA8B373: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_ptr>::reserve(unsigned int, bool) (vec.h:1858)
    -   by 0xA8B277: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_ptr>::safe_push(class_decl_loc_t::class_key_loc_t const&) (vec.h:1967)
    -   by 0xA57481: class_decl_loc_t::add_or_diag_mismatched_tag(tree_node*, tag_types, bool, bool) (parser.c:32967)
    -   by 0xA573E1: class_decl_loc_t::add(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32941)
    -   by 0xA56C52: cp_parser_check_class_key(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32819)
    -   by 0xA3AD12: cp_parser_elaborated_type_specifier(cp_parser*, bool, bool) (parser.c:20227)
    -   by 0xA37EF2: cp_parser_type_specifier(cp_parser*, int, cp_decl_specifier_seq*, bool, int*, bool*) (parser.c:18942)
    -   by 0xA31CDD: cp_parser_decl_specifier_seq(cp_parser*, int, cp_decl_specifier_seq*, int*) (parser.c:15517)
    -   by 0xA49508: cp_parser_member_declaration(cp_parser*) (parser.c:26440)
    -
    -728 bytes in 7 blocks are definitely lost in loss record 455 of 519
    -   at 0x483B7F3: malloc (vg_replace_malloc.c:309)
    -   by 0x223B63F: xrealloc (xmalloc.c:177)
    -   by 0xA8D848: void va_heap::reserve<class_decl_loc_t::class_key_loc_t>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int, bool) (vec.h:290)
    -   by 0xA8B373: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_ptr>::reserve(unsigned int, bool) (vec.h:1858)
    -   by 0xA57508: class_decl_loc_t::add_or_diag_mismatched_tag(tree_node*, tag_types, bool, bool) (parser.c:32980)
    -   by 0xA573E1: class_decl_loc_t::add(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32941)
    -   by 0xA56C52: cp_parser_check_class_key(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32819)
    -   by 0xA48BC6: cp_parser_class_head(cp_parser*, bool*) (parser.c:26090)
    -   by 0xA4674B: cp_parser_class_specifier_1(cp_parser*) (parser.c:25302)
    -   by 0xA47D76: cp_parser_class_specifier(cp_parser*) (parser.c:25680)
    -   by 0xA37E27: cp_parser_type_specifier(cp_parser*, int, cp_decl_specifier_seq*, bool, int*, bool*) (parser.c:18912)
    -   by 0xA31CDD: cp_parser_decl_specifier_seq(cp_parser*, int, cp_decl_specifier_seq*, int*) (parser.c:15517)
    -
    -832 bytes in 8 blocks are definitely lost in loss record 458 of 519
    -   at 0x483B7F3: malloc (vg_replace_malloc.c:309)
    -   by 0x223B63F: xrealloc (xmalloc.c:177)
    -   by 0xA8D848: void va_heap::reserve<class_decl_loc_t::class_key_loc_t>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int, bool) (vec.h:290)
    -   by 0xA901ED: bool vec_safe_reserve<class_decl_loc_t::class_key_loc_t, va_heap>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int, bool) (vec.h:697)
    -   by 0xA8F161: void vec_alloc<class_decl_loc_t::class_key_loc_t, va_heap>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int) (vec.h:718)
    -   by 0xA8D18D: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>::copy() const (vec.h:979)
    -   by 0xA8B0C3: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_ptr>::copy() const (vec.h:1824)
    -   by 0xA896B1: class_decl_loc_t::operator=(class_decl_loc_t const&) (parser.c:32697)
    -   by 0xA571FD: class_decl_loc_t::add(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32899)
    -   by 0xA56C52: cp_parser_check_class_key(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32819)
    -   by 0xA3AD12: cp_parser_elaborated_type_specifier(cp_parser*, bool, bool) (parser.c:20227)
    -   by 0xA37EF2: cp_parser_type_specifier(cp_parser*, int, cp_decl_specifier_seq*, bool, int*, bool*) (parser.c:18942)
    -
    -1,144 bytes in 11 blocks are definitely lost in loss record 466 of 519
    -   at 0x483B7F3: malloc (vg_replace_malloc.c:309)
    -   by 0x223B63F: xrealloc (xmalloc.c:177)
    -   by 0xA8D848: void va_heap::reserve<class_decl_loc_t::class_key_loc_t>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int, bool) (vec.h:290)
    -   by 0xA901ED: bool vec_safe_reserve<class_decl_loc_t::class_key_loc_t, va_heap>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int, bool) (vec.h:697)
    -   by 0xA8F161: void vec_alloc<class_decl_loc_t::class_key_loc_t, va_heap>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int) (vec.h:718)
    -   by 0xA8D18D: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>::copy() const (vec.h:979)
    -   by 0xA8B0C3: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_ptr>::copy() const (vec.h:1824)
    -   by 0xA896B1: class_decl_loc_t::operator=(class_decl_loc_t const&) (parser.c:32697)
    -   by 0xA571FD: class_decl_loc_t::add(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32899)
    -   by 0xA56C52: cp_parser_check_class_key(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32819)
    -   by 0xA48BC6: cp_parser_class_head(cp_parser*, bool*) (parser.c:26090)
    -   by 0xA4674B: cp_parser_class_specifier_1(cp_parser*) (parser.c:25302)
    -
    -1,376 bytes in 10 blocks are definitely lost in loss record 467 of 519
    -   at 0x483DFAF: realloc (vg_replace_malloc.c:836)
    -   by 0x223B62C: xrealloc (xmalloc.c:179)
    -   by 0xA8D848: void va_heap::reserve<class_decl_loc_t::class_key_loc_t>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int, bool) (vec.h:290)
    -   by 0xA8B373: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_ptr>::reserve(unsigned int, bool) (vec.h:1858)
    -   by 0xA8B277: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_ptr>::safe_push(class_decl_loc_t::class_key_loc_t const&) (vec.h:1967)
    -   by 0xA57481: class_decl_loc_t::add_or_diag_mismatched_tag(tree_node*, tag_types, bool, bool) (parser.c:32967)
    -   by 0xA573E1: class_decl_loc_t::add(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32941)
    -   by 0xA56C52: cp_parser_check_class_key(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32819)
    -   by 0xA3AD12: cp_parser_elaborated_type_specifier(cp_parser*, bool, bool) (parser.c:20227)
    -   by 0xA37EF2: cp_parser_type_specifier(cp_parser*, int, cp_decl_specifier_seq*, bool, int*, bool*) (parser.c:18942)
    -   by 0xA31CDD: cp_parser_decl_specifier_seq(cp_parser*, int, cp_decl_specifier_seq*, int*) (parser.c:15517)
    -   by 0xA301E0: cp_parser_simple_declaration(cp_parser*, bool, tree_node**) (parser.c:14772)
    -
    -3,552 bytes in 33 blocks are definitely lost in loss record 483 of 519
    -   at 0x483B7F3: malloc (vg_replace_malloc.c:309)
    -   by 0x223B63F: xrealloc (xmalloc.c:177)
    -   by 0xA8D848: void va_heap::reserve<class_decl_loc_t::class_key_loc_t>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int, bool) (vec.h:290)
    -   by 0xA901ED: bool vec_safe_reserve<class_decl_loc_t::class_key_loc_t, va_heap>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int, bool) (vec.h:697)
    -   by 0xA8F161: void vec_alloc<class_decl_loc_t::class_key_loc_t, va_heap>(vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>*&, unsigned int) (vec.h:718)
    -   by 0xA8D18D: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_embed>::copy() const (vec.h:979)
    -   by 0xA8B0C3: vec<class_decl_loc_t::class_key_loc_t, va_heap, vl_ptr>::copy() const (vec.h:1824)
    -   by 0xA8964A: class_decl_loc_t::class_decl_loc_t(class_decl_loc_t const&) (parser.c:32689)
    -   by 0xA8F515: hash_table<hash_map<tree_decl_hash, class_decl_loc_t, simple_hashmap_traits<default_hash_traits<tree_decl_hash>, class_decl_loc_t> >::hash_entry, false, xcallocator>::expand() (hash-table.h:839)
    -   by 0xA8D4B3: hash_table<hash_map<tree_decl_hash, class_decl_loc_t, simple_hashmap_traits<default_hash_traits<tree_decl_hash>, class_decl_loc_t> >::hash_entry, false, xcallocator>::find_slot_with_hash(tree_node* const&, unsigned int, insert_option) (hash-table.h:1008)
    -   by 0xA8B1DC: hash_map<tree_decl_hash, class_decl_loc_t, simple_hashmap_traits<default_hash_traits<tree_decl_hash>, class_decl_loc_t> >::get_or_insert(tree_node* const&, bool*) (hash-map.h:200)
    -   by 0xA57128: class_decl_loc_t::add(cp_parser*, unsigned int, tag_types, tree_node*, bool, bool) (parser.c:32888)
     [...]
     LEAK SUMMARY:
    -   definitely lost: 8,440 bytes in 81 blocks
    +   definitely lost: 48 bytes in 1 blocks
        indirectly lost: 12,529 bytes in 329 blocks
          possibly lost: 0 bytes in 0 blocks
        still reachable: 1,644,376 bytes in 768 blocks

	gcc/
	* hash-table.h (hash_table<Descriptor, Lazy, Allocator>::expand):
	Destruct stale Value objects.
	* hash-map-tests.c (test_map_of_type_with_ctor_and_dtor_expand):
	Update.