c++: NRV and goto [PR92407]

c++: NRV and goto [PR92407]

Here our named return value optimization was breaking the required
destructor when the goto takes 'a' out of scope.  A simple fix for the
release branches is to disable the optimization in the presence of backward
goto.

We could do better by disabling the optimization only if there is a backward
goto across the variable declaration, but we don't track that, and in GCC 14
we instead make the goto work with NRV.

	PR c++/92407

gcc/cp/ChangeLog:

	* cp-tree.h (struct language_function): Add backward_goto.
	* decl.cc (check_goto): Set it.
	* typeck.cc (check_return_expr): Prevent NRV if set.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/nrv22.C: New test.