[C++ PATCH] use hash-table for namespace contents

[C++ PATCH] use hash-table for namespace contents

https://gcc.gnu.org/ml/gcc-patches/2017-10/msg00365.html
	Use hash_table for namespace bindings
	* cp-tree.h (struct named_decl_hash): New.
	(lang_decl_ns): Change type of bindings field.
	* lex.c (maybe_add_lang_decl_raw): Adjust.
	* name-lookup.c (find_namespace_slot): Adjust.
	(do_pushdecl): Push NULL-named namespace.
	(do_push_nested_namespace): Adjust.
	(push_namespace): Push anonymous namespace as NULL name.

From-SVN: r253489