c++ modules: partial variable template specializations [PR106826]

With partial variable template specializations, it looks like we
stream the VAR_DECL (i.e. the DECL_TEMPLATE_RESULT of the corresponding
TEMPLATE_DECL) since process_partial_specialization adds it to the
specializations table, but we end up never streaming the corresponding
TEMPLATE_DECL itself that's reachable only from the primary template's
DECL_TEMPLATE_SPECIALIZATIONS list, which leads to this list being
incomplete on stream-in.

The modules machinery already has special logic for streaming partial
specializations of class templates; this patch attempts to generalize
it to handle those of variable templates as well.

	PR c++/106826

gcc/cp/ChangeLog:

	* module.cc (trees_out::decl_value): Use get_template_info in
	the MK_partial case to handle both VAR_DECL and TYPE_DECL.
	(trees_out::key_mergeable): Likewise.
	(trees_in::key_mergeable): Likewise.
	(has_definition): Consider DECL_INITIAL of a partial variable
	template specialization.
	(depset::hash::make_dependency): Handle partial variable template
	specializations too.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/partial-2_a.C: New test.
	* g++.dg/modules/partial-2_b.C: New test.