Make typing into incomplete C++ raw strings work, and make it work fast enough

Make typing into incomplete C++ raw strings work, and make it work fast enough

* lisp/progmodes/cc-engine.el (c-beginning-of-macro, c-state-pp-to-literal):
Put `save-match-data' around calls to `looking-at' to enable the use of the
match data in higher level functions.

* lisp/progmodes/cc-fonts.el (c-font-lock-declarations)
(c-font-lock-cut-off-declarators): Use `limit' rather than `(point-max)' as a
limit to speed up handling of C++ raw strings.