objc: Add support for per-location warning groups.

gcc/objc/ChangeLog:

	* objc-act.c (objc_maybe_build_modify_expr): Replace direct uses
	of TREE_NO_WARNING with warning_suppressed_p, and suppress_warning.
	(objc_build_incr_expr_for_property_ref): Same.
	(objc_build_struct): Same.
	(synth_module_prologue): Same.
	* objc-gnu-runtime-abi-01.c (gnu_runtime_01_initialize): Same.
	* objc-next-runtime-abi-01.c (next_runtime_01_initialize): Same.
	* objc-next-runtime-abi-02.c (next_runtime_02_initialize): Same.