c++: Extend the PR96204 fix to variable templates too

c++: Extend the PR96204 fix to variable templates too

r12-1829 corrected the access scope during partial specialization
matching of class templates, but overlooked the variable template case.
This patch moves the access scope adjustment to within
most_specialized_partial_spec so that all callers can benefit.

This patch also adjusts a couple of these callers to avoid always
passing the most general template of a variable template specialization,
since that'd cause us to push the wrong access scope for e.g. the second
testcase below (we'd push A<T> instead of A<int>/A<char>).  We ought to
be passing the partially instantiated template instead.

	PR c++/96204

gcc/cp/ChangeLog:

	* pt.c (finish_template_variable): Pass the partially
	instantiated template and its args to instantiate_template.
	(instantiate_class_template_1): No need to call
	push_nested_class and pop_nested_class around the call to
	most_specialized_partial_spec.
	(instantiate_template_1): Pass the partially instantiated
	template to lookup_template_variable.
	(most_specialized_partial_spec):  Use push_access_scope_guard
	to set the access scope appropriately.  Use
	deferring_access_check_sentinel to force access to get checked
	immediately.
	(instantiate_decl): Just pass the VAR_DECL to
	most_specialized_partial_spec.

gcc/testsuite/ChangeLog:

	* g++.dg/template/access41.C: New test.
	* g++.dg/template/access41a.C: New test.