c++: Implement resolution for DR 36 [PR116160]

c++: Implement resolution for DR 36 [PR116160]

This implements part of P1787 to no longer complain about redeclaring an
entity via using-decl other than in a class scope.

	PR c++/116160

gcc/cp/ChangeLog:

	* name-lookup.cc (supplement_binding): Allow redeclaration via
	USING_DECL if not in class scope.
	(do_nonmember_using_decl): Remove function-scope exemption.
	(push_using_decl_bindings): Remove outdated comment.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/using-enum-3.C: No longer expect an error.
	* g++.dg/lookup/using53.C: Remove XFAIL.
	* g++.dg/cpp2a/using-enum-11.C: New test.

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