c++: covariant reference return types [PR99664]

c++: covariant reference return types [PR99664]

This implements the wording changes of CWG 960 which clarifies that two
reference types are covariant only if they're both lvalue references
or both rvalue references.

	DR 960
	PR c++/99664

gcc/cp/ChangeLog:

	* search.c (check_final_overrider): Compare TYPE_REF_IS_RVALUE
	when the return types are references.

gcc/testsuite/ChangeLog:

	* g++.dg/inherit/covariant23.C: New test.