c++: allow NRV and non-NRV returns [PR58487]

c++: allow NRV and non-NRV returns [PR58487]

Now that we support NRV from an inner block, we can also support non-NRV
returns from other blocks, since once the NRV is out of scope a later return
expression can't possibly alias it.

This fixes 58487 and half-fixes 53637: now one of the returns is elided, but
not the other.

Fixing the remaining xfails in these testcases will require a very different
approach, probably involving a full tree/block walk from finalize_nrv, and
check_return_expr only adding to a list of potential return variables.

	PR c++/58487
	PR c++/53637

gcc/cp/ChangeLog:

	* cp-tree.h (INIT_EXPR_NRV_P): New.
	* semantics.cc (finalize_nrv_r): Check it.
	* name-lookup.h (decl_in_scope_p): Declare.
	* name-lookup.cc (decl_in_scope_p): New.
	* typeck.cc (check_return_expr): Allow non-NRV
	returns if the NRV is no longer in scope.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/nrv26.C: New test.
	* g++.dg/opt/nrv26a.C: New test.
	* g++.dg/opt/nrv27.C: New test.