c++: Prevent dangling pointers from becoming nullptr in constexpr [PR110619]

Currently, when typeck discovers that a return statement will refer to a
local variable it rewrites to return a null pointer. This causes the
error messages for using the return value in a constant expression to be
unhelpful, especially for reference return values, and is also a visible
change to otherwise valid code (as in the linked PR).

The transformation is nonetheless important, however, both as a safety
guard against attackers being able to gain a handle to other data on the
stack, and to prevent duplicate warnings from later null-dereference
warning passes.

As such, this patch just delays the transformation until cp_genericize,
after constexpr function definitions have been generated.

	PR c++/110619

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_genericize_r): Transform RETURN_EXPRs to
	not return dangling pointers.
	* cp-tree.h (RETURN_EXPR_LOCAL_ADDR_P): New flag.
	(check_return_expr): Add a new parameter.
	* semantics.cc (finish_return_stmt): Set flag on RETURN_EXPR
	when referring to dangling pointer.
	* typeck.cc (check_return_expr): Disable transformation of
	dangling pointers, instead pass this information to caller.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-110619.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>