Implement C23 features:

Implement C23 features:

Improve the handling of #error and #warning in C and C++ Modes.

* lisp/progmodes/cc-engine.el (c-looking-at-c++-attribute)
(c-enclosing-c++-attribute, c-forward-sws, c-backward-sws):
Handle attributes in C as they have been handled in C++.
(c-forward-align-clause-throw-if-invalid): New macro.
(c-forward-type): Handle alignas, alignof, and _BitInt, putting
a catch block around most of the function to catch invalid uses
of these new keywords.
(c-in-id-arglist): New function.
(c-forward-decl-or-cast-1): Recognize a "maybe" type identifier
as a type in arglists when there is no parameter identifier
associated with it.

* lisp/progmodes/cc-fonts.el (c-font-lock-cpp-messages): New
function.
(c-cpp-matchers): Move the handling of "invalid" comment
delimiters outside of the block handling CPP directives.
Remove the inline handling of #error and #warning, using the
new function c-font-lock-cpp-messages instead.
(c-get-fontification-context): Handle alignof, alignas,
_BitInt.
(c-font-lock-declarations): Adapt fontification of K&R
parameters to the C23 nameless parameter scheme.

* lisp/progmodes/cc-langs.el (c-has-quoted-numbers)
(c-stmt-boundary-skip-chars)
(c-recognize-post-brace-list-type-p, c-modifier-kwds)
(c-constant-kwds): Handle C the same as the existing C++
handling.
(c-cpp-message-directives-re, noncontinued-line-end)
(c-cpp-messages-re, c-cpp-message-match-no): New
c-lang-consts/vars.
(c-cpp-include-directives): New directive embed.
(c-cpp-expr-directives): New directives elifdef, elifndef.
(c-primitive-type-kwds): New types, _Decimal*, bool, char*_t,
nullptr_t.
(c-typeof-kwds): New keyword typeof_unqual.
(c-type-with-paren-kwds, c-type-with-paren-key): New
c-lang-const/vars.
(c-type-modifier-with-parens-kwds)
(c-type-modifier-with-parens-key, c-type-internal-paren-kwds)
(c-type-internal-paren-key): New c-lang-const/vars.
(c-type-modifier-prefix-kwds, c-type-start-kwds): Amend with
the above new c-lang-consts.
(c-no-type-with-equals-kwds, c-no-type-with-equals-key): New
c-lang-const/vars.
(c-modifier-kwds): Add constexpr, auto, and thread_local to the
C value.
(c-paren-nontype-kwds): Add static_assert to the C value.
(c-constant-kwds): Add nullptr to the C value.
(c-regular-keywords-regexp): Include c-type-with-paren-kwds.
(c-recognize-nameless-type-decls): New c-lang-const/var.

* lisp/progmodes/cc-mode.el (c-leave-cc-mode-mode): Also clear
c-digit-separator properties.

(cherry picked from commit 8ddb8b0e45f150a5866c86af8ed2054b792761ff)