c++/modules: Find non-exported reachable decls when instantiating friend classes...

c++/modules: Find non-exported reachable decls when instantiating friend classes [PR119863]

In r15-9029-geb26b667518c95, we started checking for conflicting
declarations with any reachable decl attached to the same originating
module.  This exposed the issue in the PR, where we would always create
a new type even if a matching type existed in the original module.

This patch reworks lookup_imported_hidden_friend to handle this case
better, by first checking for any reachable decl in the attached module
before looking in the mergeable decl slots.

	PR c++/119863

gcc/cp/ChangeLog:

	* name-lookup.cc (get_mergeable_namespace_binding): Remove
	no-longer-used function.
	(lookup_imported_hidden_friend): Also look for hidden imported
	decls in an attached decl's module.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/tpl-friend-18_a.C: New test.
	* g++.dg/modules/tpl-friend-18_b.C: New test.
	* g++.dg/modules/tpl-friend-18_c.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>