C-family, Objective-C [1/3] : Implement Wobjc-root-class [PR77404].

This warning catches the case that the user has left the
superclass specification from a class interface.  Root
classes are, of course, permitted and an attribute is added
to mark these so that the diagnostic is suppressed.

The warning and attribute spellings have been kept in sync
with the language reference implementation (clang).

The diagnostic location information present in the objective-c
interface and class definitions is relatively poor.  This patch
adds a location for the class name to the interface and makes use
of it in existing warnings.

Part 1 is the changes to code and added tests.

Many entries in the testsuite make use of root classes so
there are a large number of mechanical changes there adding
"-Wno-objc-root-class" to the options.

The test changes are parts 2 (objective-c) and 3 (objective-c++)
in the patch series.

gcc/c-family/ChangeLog:

	PR objc/77404
	* c-attribs.c (handle_objc_root_class_attribute): New
	* c-objc.h (objc_start_class_interface): Add a location
	value for the position of the class name.
	* c.opt: Add Wobjc-root-class.
	* stub-objc.c (objc_start_class_interface): Add a location
	value for the position of the class name.

gcc/c/ChangeLog:

	PR objc/77404
	* c-parser.c (c_parser_objc_class_definition): Pass the
	location of the class name to the interface declaration.

gcc/cp/ChangeLog:

	PR objc/77404
	* parser.c (cp_parser_objc_class_interface): Pass the
	location of the class name to the interface declaration.

gcc/objc/ChangeLog:

	PR objc/77404
	* objc-act.c (objc_start_class_interface): Accept the location
	of the class name, use it in existing diagnostic.
	(start_class): Accept obj_root_class type attributes.  Warn when
	the interface for an implementation does not contain a super
	class (unless the diagnostic is suppressed by the the command
	line flag or the objc_root_class type attribute).

gcc/testsuite/ChangeLog:

	PR objc/77404
	* objc.dg/attributes/root-class-01.m: New test.
	* objc.dg/root-class-00.m: New test.
	* obj-c++.dg/attributes/root-class-01.mm: New test.
	* obj-c++.dg/root-class-00.mm: New test.

gcc/ChangeLog:

	PR objc/77404
	* doc/extend.texi: Document the objc_root_class attribute.
	* doc/invoke.texi: Document -Wobjc-root-class.