c++/modules: Fix linkage checks for exported using-decls

c++/modules: Fix linkage checks for exported using-decls

This fixes some inconsistencies with what kinds of linkage various
entities are assumed to have.  This also fixes handling of exported
using-decls binding to GM entities and type aliases to better align with
the standard's requirements.

gcc/cp/ChangeLog:

	* name-lookup.cc (check_can_export_using_decl): Handle internal
	linkage GM entities (but ignore in header units); use linkage
	of entity ultimately referred to by aliases.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/using-10.C: Add tests for no-linkage, fix
	expected linkage of aliases.
	* g++.dg/modules/using-12.C: Likewise.
	* g++.dg/modules/using-27.C: New test.
	* g++.dg/modules/using-28_a.C: New test.
	* g++.dg/modules/using-28_b.C: New test.
	* g++.dg/modules/using-29.H: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>