c++: memfn lookup consistency and dependent using-decls

c++: memfn lookup consistency and dependent using-decls

Rather than not doing any filtering when filter_memfn_lookup encounters
a dependent using-decl, handle this case less imprecisely by holding on
to the members in the new lookup set that come from a base, i.e. that
could plausibly have been introduced by that using-decl, and filtering
the rest as usual.  This is still imperfect, but it's closer to the
correct answer than the previous behavior was.

gcc/cp/ChangeLog:

	* pt.cc (filter_memfn_lookup): Handle dependent USING_DECL
	better.