c++: modularize spelling suggestions

c++: modularize spelling suggestions

This augments the spelling suggestion code to understand about visible
imported modules.  Simply consider each visible binding in the
binding_vector, until we find one that has something of interest.

	gcc/cp/
	* name-lookup.c: Include bitmap.h.
	(enum binding_slots): New.
	(maybe_add_fuzzy_binding): Return bool true if found.
	(consider_binding_level): Add module support.
	* module.cc (get_import_bitmap): Stub.