CC Mode: Prevent "const" inside an identifier being recognized as the keyword

CC Mode: Prevent "const" inside an identifier being recognized as the keyword

This fixes bug #45560.

* lisp/progmodes/cc-engine.el (c-forward-declarator)
(c-forward-decl-or-cast-1): Amend certain regexp match numbers on account of
the change below.  Surround some looking-at calls with save-match-data.

* lisp/progmodes/cc-langs.el (c-type-decl-prefix-keywords-key): New lang
const.
(c-type-decl-prefix-key): Reformulate to match operators and keywords
separately, using the new lang const (above).