c++: __is_constructible ref binding [PR100667]

c++: __is_constructible ref binding [PR100667]

The requirement that a type argument be complete is excessive in the case of
direct reference binding to the same type, which does not rely on any
properties of the type.  This is LWG 2939.

	PR c++/100667

gcc/cp/ChangeLog:

	* semantics.cc (same_type_ref_bind_p): New.
	(finish_trait_expr): Use it.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_constructible8.C: New test.