c++: access scope during partial spec matching [PR96204]

Here, when determining whether the partial specialization matches
has_type_member<Child>, we do so from the scope of where the template-id
appears rather than from the scope of the specialization, and this
causes us to select the partial specialization (since Child::type is
accessible from Parent).  When we later instantiate this partial
specialization, we've entered the scope of the specialization and so
substitution into e.g. the DECL_CONTEXT of has_type_member::value fails
with access errors since the friend declaration that we relied on to
choose the partial specialization no longer applies.

It seems the appropriate access scope from which to perform partial
specialization matching is the specialization itself (similar to how
we check access of base-clauses), which is what this patch implements.

	PR c++/96204

gcc/cp/ChangeLog:

	* pt.c (instantiate_class_template_1): Enter the scope of the
	type when calling most_specialized_partial_spec.

gcc/testsuite/ChangeLog:

	* g++.dg/template/access40.C: New test.
	* g++.dg/template/access40a.C: New test.