c, analyzer: fix ICE with -fanalyzer and -Wunused-macros [PR107711]

PR analyzer/107711 reports an ICE since r13-4073-gd8aba860b34203 with
the combination of -fanalyzer and -Wunused-macros.

The issue is that in c_translation_unit::consider_macro's call to
cpp_create_reader I was passing "ident_hash" for use by the the new
reader, but that takes ownership of that hash_table, so that ident_hash
erroneously gets freed when c_translation_unit::consider_macro calls
cpp_destroy, leading to a use-after-free in -Wunused-macros, where:

(gdb) p pfile->hash_table->pfile == pfile
$23 = false

and it's instead pointing at the freed reader from consider_macro,
leading to a use-after-free ICE.

Fixed thusly.

gcc/c/ChangeLog:
	PR analyzer/107711
	* c-parser.cc (ana::c_translation_unit::consider_macro): Pass NULL
	to cpp_create_reader, rather than ident_hash, so that the new
	reader gets its own hash table.

gcc/testsuite/ChangeLog:
	PR analyzer/107711
	* gcc.dg/analyzer/named-constants-Wunused-macros.c: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>