c++: using overloaded with local decl [PR92918]

The problem here was that the lookup for 'impl' when parsing the template
only found the using-declaration, not the member function declaration.

This happened because when trying to add the member function declaration,
push_class_level_binding_1 saw that the current binding was a USING_DECL and
the new value is an overload, and decided to just return success.

That 'return true' dates back to r69921.  In
https://gcc.gnu.org/pipermail/gcc-patches/2003-July/110632.html Nathan
mentions that we only push dependent USING_DECLs, which is no longer the
case; now that we retain more USING_DECLs, handling this case like the other
overloaded function cases seems like the obvious solution.

gcc/cp/ChangeLog:

	PR c++/92918
	* name-lookup.c (push_class_level_binding_1): Do overload a new
	function with a previous using-declaration.

gcc/testsuite/ChangeLog:

	PR c++/92918
	* g++.dg/lookup/using66.C: New test.