c++: Consistently expose singleton overloads

This is a patch from my name-lookup overhaul.  I noticed the parser
and one path in name-lookup looked through an overload of a single
known decl.  It seems more consistent to do that in both paths through
name-lookup, and not in the parser itself.

	gcc/cp/
	* name-lookup.c (lookup_qualified_name): Expose an overload of a
	singleton with known type.
	(lookup_name_1): Just check the overload's type to expose it.
	* parser.c (cp_parser_lookup_name): Do not do that check here.