c: add Wzero-as-null-pointer-constant [PR117059]

Add warnings for the use of zero as a null pointer constant to the C FE.

	PR c/117059

gcc/c-family/ChangeLog:
	* c.opt (Wzero-as-null-pointer-constant): Enable for C and ObjC.

gcc/c/ChangeLog:
	* c-typeck.cc (parse_build_binary_op): Add warning.
	(build_conditional_expr): Add warning.
	(convert_for_assignment): Add warning.

gcc/ChangeLog:
	* doc/invoke.texi (Wzero-as-null-pointer-constant): Adapt
	description.

gcc/testsuite/ChangeLog:
	* gcc.dg/Wzero-as-null-pointer-constant.c: New test.

Suggested-by: Alejandro Colomar <alx@kernel.org>
Acked-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Joseph Myers <josmyers@redhat.com>