CC Mode: Fix wrong value of comment-start-skip, fixing half of bug #41952

CC Mode: Fix wrong value of comment-start-skip, fixing half of bug #41952

Also add functions to enable correct use of CC Mode's filling functionality
from major modes which don't initialize CC Mode fully.  These modes are
currently js-mode and mhtml-mode.

* lisp/progmodes/cc-langs.el (comment-start-skip): Replace "\\(" by "\\(?:" so
that (match-end 1) isn't falsely taken to be the start of the comment.

* lisp/progmodes/cc-engine.el (c-foreign-truncate-lit-pos-cache)
(c-foreign-init-lit-pos-cache): New functions.