c++: name-lookup cleanups

c++: name-lookup cleanups

Name-lookup is the most changed piece of the front end for modules.
Here are some preparatort cleanups and API extensions.

	gcc/cp/
	* name-lookup.h (set_class_bindings): Return vector, take signed
	'extra' parm.
	* name-lookup.c (maybe_lazily_declare): Break out ...
	(get_class_binding): .. of here, call it.
	(find_member_slot): Adjust get_class_bindings call.
	(set_class_bindings): Allow -ve extra.  Return the vector.
	(set_identifier_type_value_with_scope): Remove checking assert.
	(lookup_using_decl): Set decl's context.
	(do_pushtag): Adjust set_identifier_type_value_with_scope handling.