PR target/44981
* c-format.c (format_type): New type gcc_objc_string_format_type.
(valid_stringptr_type_p): New.
(handle_format_arg_attribute): Use valid_stringptr_type_p ().
(check_format_string): Pass expected type, use
valid_stringptr_type_p (), check that the format string types are
consistent with the format specification.
(decode_format_attr): Warn if NSString is used outside objective-c.
(format_types_orig): Add NSString.
(format_name): New.
(format_flags): New.
(check_format_arg): Handle format strings requiring an external parser.
first_target_format_type: New variable.
(handle_format_attribute): Set up first_target_format_type, pass the
expected format arg string type to check_format_string().
* c-common.h (FMT_FLAG_PARSE_ARG_CONVERT_EXTERNAL): New flag.
* stub-objc.c (objc_string_ref_type_p): New.
(objc_check_format_arg): New.