Add handling of the C11 _Generic construct to C Mode

Add handling of the C11 _Generic construct to C Mode

This fixes bug #61481.

* lisp/progmodes/cc-engine.el (c-guess-basic-syntax): Add a new CASE 6 for
_Generic.

* lisp/progmodes/cc-fonts.el (c-fontify-types-and-refs): Use `let*' rather
than `let'.
(c-get-fontification-context): Add the new result `generic', and add handling
to determine it.
(c-font-lock-declarations): Call c-font-lock-c11-generic-clause when needed.
(c-font-lock-c11-generic-clause): New function.

* lisp/progmodes/cc-langs.el (c-generic-kwds, c-generic-key): New lang
constants/variable.