Reimplement unqualified namespace lookup.

Reimplement unqualified namespace lookup.

	gcc/cp/
	Reimplement unqualified namespace lookup.
	* name-lookup.c (name_lookup::using_pair)
	name_lookup::using_queue): New typedefs.
	(name_lookup::queue_namespace, name_lookup::do_queue_usings,
	name_lookup::queue_usings): New.
	(name_lookup::search_unqualified): New.
	(merge_functions, same_entity_p, ambiguous_decl)
	unqualified_namespace_lookup_1, unqualified_namespace_lookup,
	lookup_using_namespace): Delete.
	(lookup_name_real_1): Adjust.

	gcc/testsuite/
	* g++.dg/lookup/using17.C: Adjust diagnostics.

From-SVN: r248465