[C++ PATCH] namespace using directives

[C++ PATCH] namespace using directives

https://gcc.gnu.org/ml/gcc-patches/2019-05/msg01245.html
	* cp-tree.h (struct lang_decl_ns): Remove usings field.
	(DECL_NAMESPACE_USING): Delete.
	* name-lookup.c (name_lookup::search_usings): Use namespace's
	binding scope.
	(name_lookup::queue_namespae): Likewise.
	(finish_namespace_using_directive, push_namespace): Likewise.
	(has_using_namespace_std_directive): Just search the entire
	binding stack.

From-SVN: r271416