c++: Add -Wnrvo

c++: Add -Wnrvo

While looking at PRs about cases where we don't perform the named return
value optimization, it occurred to me that it might be useful to have a
warning for that.

This does not fix PR58487, but might be interesting to people watching it.

	PR c++/58487

gcc/c-family/ChangeLog:

	* c.opt: Add -Wnrvo.

gcc/ChangeLog:

	* doc/invoke.texi: Document it.

gcc/cp/ChangeLog:

	* typeck.cc (want_nrvo_p): New.
	(check_return_expr): Handle -Wnrvo.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/nrv25.C: New test.