c: Reject _Atomic type * as last argument to __builtin_*_overflow [PR90628]

c: Reject _Atomic type * as last argument to __builtin_*_overflow [PR90628]

During the __builtin_clear_padding implementation, I've noticed we don't
diagnose _Atomic whatever * as last argument to __builtin_*_overflow.
As the storing by that builtin isn't atomic in any way, I think we should
reject it.

2020-11-17  Jakub Jelinek  <jakub@redhat.com>

	PR c/90628
	* c-common.c (check_builtin_function_arguments)
	<case BUILT_IN_ADD_OVERFLOW>: Diagnose when last argument is pointer
	to _Atomic.  For the TYPE_READONLY case, adjust message to be usable
	for more builtins and argument positions.

	* gcc.dg/builtin-arith-overflow-4.c: New test.