c++/modules: using builtin

c++/modules: using builtin

Here, when we try to bring "memset" back into the global namespace, we find
the built-in, see that it's the same declaration (because the module brought
it into the other namespace with a using-declaration), and decide that we
don't need to do anything.  But we still need a non-hidden overload.

gcc/cp/ChangeLog:

	* name-lookup.cc (do_nonmember_using_decl): Handle hidden better.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/using-33_a.C: New test.
	* g++.dg/modules/using-33_b.C: New test.