c++: fix __is_invocable for std::reference_wrapper [PR121055]

c++: fix __is_invocable for std::reference_wrapper [PR121055]

Our implementation of the INVOKE spec ([func.require]) was incorrectly
treating reference_wrapper<T>::get() as returning T instead of T&, which
notably makes a difference when invoking a ref-qualified memfn pointer.

	PR c++/121055

gcc/cp/ChangeLog:

	* method.cc (build_invoke): Correct reference_wrapper handling.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_invocable5.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>