c++: visibility wrt concept-id as targ [PR115283]

c++: visibility wrt concept-id as targ [PR115283]

Like with alias templates, it seems we don't maintain visibility flags
for concepts either, so min_vis_expr_r should ignore them for now.
Otherwise after r14-6789 we may incorrectly give a function template that
uses a concept-id in its signature internal linkage.

	PR c++/115283

gcc/cp/ChangeLog:

	* decl2.cc (min_vis_expr_r) <case TEMPLATE_DECL>: Ignore
	concepts.

gcc/testsuite/ChangeLog:

	* g++.dg/template/linkage5.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>