c: Add __typeof_unqual__ and __typeof_unqual support

As I mentioned in my stdckdint.h mail, I think having __ prefixed
keywords for the typeof_unqual keyword which can be used in earlier
language modes can be useful, not all code can be switched to C23
right away.

The following patch implements that.  It keeps the non-C23 behavior
for it for the _Noreturn functions to stay compatible with how
__typeof__ behaves.

I think we don't need it for C++, in C++ we have standard
traits to remove qualifiers etc.

2023-08-11  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* doc/extend.texi (Typeof): Document typeof_unqual
	and __typeof_unqual__.
gcc/c-family/
	* c-common.cc (c_common_reswords): Add __typeof_unqual
	and __typeof_unqual__ spellings of typeof_unqual.
gcc/c/
	* c-parser.cc (c_parser_typeof_specifier): Handle
	__typeof_unqual and __typeof_unqual__ as !is_std.
gcc/testsuite/
	* gcc.dg/c11-typeof-2.c: New test.
	* gcc.dg/c11-typeof-3.c: New test.
	* gcc.dg/gnu11-typeof-3.c: New test.
	* gcc.dg/gnu11-typeof-4.c: New test.