re PR c++/77591 (decltype(auto) and ternary operator allow returning local reference without a warning)

	PR c++/77591
	* typeck.c (maybe_warn_about_returning_address_of_local): Optimize
	whats_returned through fold_for_warn.

	* g++.dg/cpp1y/pr77591.C: New test.

From-SVN: r242924