c++: visibility wrt template and ptrmem targs [PR70413]

c++: visibility wrt template and ptrmem targs [PR70413]

When constraining the visibility of an instantiation, we weren't
properly considering the visibility of PTRMEM_CST and TEMPLATE_DECL
template arguments.

This patch fixes this.  It turns out we don't maintain the relevant
visibility flags for alias templates (e.g. TREE_PUBLIC is never set),
so continue to ignore alias template template arguments for now.

	PR c++/70413
	PR c++/107906

gcc/cp/ChangeLog:

	* decl2.cc (min_vis_expr_r): Handle PTRMEM_CST and TEMPLATE_DECL
	other than those for alias templates.

gcc/testsuite/ChangeLog:

	* g++.dg/template/linkage2.C: New test.
	* g++.dg/template/linkage3.C: New test.
	* g++.dg/template/linkage4.C: New test.
	* g++.dg/template/linkage4a.C: New test.