c++: prvalue array decay [PR94264]

c++: prvalue array decay [PR94264]

My change for PR53220 made array to pointer decay for prvalue arrays
ill-formed to catch well-defined C code that produces a dangling pointer in
C++ due to the shorter lifetime of compound literals.  This wasn't really
correct, but wasn't a problem until C++17 added prvalue arrays, at which
point it started rejecting valid C++ code.

I wanted to make sure that we still diagnose the problematic code;
-Wdangling-pointer covers the array-lit.c case, but I needed to extend
-Wreturn-local-addr to handle the return case.

	PR c++/94264
	PR c++/53220

gcc/c/ChangeLog:

	* c-typeck.cc (array_to_pointer_conversion): Adjust -Wc++-compat
	diagnostic.

gcc/cp/ChangeLog:

	* call.cc (convert_like_internal): Remove obsolete comment.
	* typeck.cc (decay_conversion): Allow array prvalue.
	(maybe_warn_about_returning_address_of_local): Check
	for returning pointer to temporary.

gcc/testsuite/ChangeLog:

	* c-c++-common/array-lit.c: Adjust.
	* g++.dg/cpp1z/array-prvalue1.C: New test.
	* g++.dg/ext/complit17.C: New test.