libstdc++: Unordered containers merge re-use hash code

When merging 2 unordered containers with same hasher we can re-use the hash code from
the cache if any.

Also in the context of the merge operation on multi-container use previous insert iterator as a hint
for the next insert.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h:
	(_Hash_code_base<>::_M_hash_code(const _Hash&, const _Hash_node_value<_Value, true>&)): New.
	(_Hash_code_base<>::_M_hash_code<_H2>(const _H2&, const _Hash_node_value<>&)): New.
	* include/bits/hashtable.h (_Hashtable<>::_M_merge_unique): Use latter.
	(_Hashtable<>::_M_merge_multi): Likewise.
	* testsuite/23_containers/unordered_multiset/modifiers/merge.cc (test05): New test.
	* testsuite/23_containers/unordered_set/modifiers/merge.cc (test04): New test.