c++: Incorrect specialization hash table [PR 99285]

c++: Incorrect specialization hash table [PR 99285]

Class template partial specializations need to be in the
specialization hash, but not all of them.  This defers adding
streamed-in entities to the hash table, in the same way I deferred
adding the instantiation and specialization lists for 99170.

	PR c++/99285
	gcc/cp/
	* cp-tree.h (match_mergeable_specialization)
	(add_mergeable_specialization): Adjust parms.
	* module.cc (trees_in::decl_value): Adjust
	add_mergeable_specialization calls.
	(trees_out::key_mergeable): Adjust match_mergeable_specialization
	calls.
	(specialization_add): Likewise.
	* pt.c (match_mergeable_specialization): Do not insert.
	(add_mergeable_specialization): Add to hash table here.
	gcc/testsuite/
	* g++.dg/modules/pr99285_a.H: New.
	* g++.dg/modules/pr99285_b.H: New.