CC Mode: Make implicit int types following specifiers fontify correctly

CC Mode: Make implicit int types following specifiers fontify correctly

This fixes bug #59267.  It applies to C Mode only, and refers to constructs
like "register count;" which are implicitly of int type.

* lisp/progmodes/cc-engine.el (c-forward-type): Use the new regexp
c-maybe-typeless-specifier-re in place of c-opt-type-modifier-prefix-key.  Add
an extra arm to the main cond form to handle the construct, and another to
handle "extern "C" foo_t my_foo;".
(c-forward-decl-or-cast-1): Adapt to handle the result no-id from
c-forward-type.

* lisp/progmodes/cc-langs.el (c-maybe-typeless-specifier-re): New lang
const/var.