c++ modules: variable template partial spec fixes [PR107033]

c++ modules: variable template partial spec fixes [PR107033]

In r13-2775-g32d8123cd6ce87 I missed that we need to adjust the call to
add_mergeable_specialization in the MK_partial case to correctly handle
variable template partial specializations (it currently assumes we're
always dealing with one for a class template).  This fixes an ICE when
converting the testcase from that commit to use an importable header
instead of a named module.

	PR c++/107033

gcc/cp/ChangeLog:

	* module.cc (trees_in::decl_value): In the MK_partial case for
	a variable template partial specialization, pass decl_p=true to
	add_mergeable_specialization, and set spec to the VAR_DECL not
	the TEMPLATE_DECL.
	* pt.cc (add_mergeable_specialization): For a variable template
	partial specialization, set the TREE_TYPE of the new
	DECL_TEMPLATE_SPECIALIZATIONS node to the TREE_TYPE of the
	VAR_DECL not the VAR_DECL itself.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/partial-2.cc, g++.dg/modules/partial-2.h: New
	files, factored out from ...
	* g++.dg/modules/partial-2_a.C, g++.dg/modules/partial-2_b.C: ...
	these.
	* g++.dg/modules/partial-2_c.H: New test.
	* g++.dg/modules/partial-2_d.C: New test.