c++: fold calls to std::forward_like [PR96780]

c++: fold calls to std::forward_like [PR96780]

This extends our folding of cast-like standard library functions
to also include C++23's std::forward_like.

	PR c++/96780

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_fold) <case CALL_EXPR>: Fold calls
	to std::forward_like as well.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/pr96780.C: Also test std::forward_like folding.

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