c-parse-state. Don't "append-lower-brace-pair" in certain

c-parse-state.  Don't "append-lower-brace-pair" in certain
circumstances.  Also fix an obscure bug where "\\s!" shouldn't be
recognised as a comment.

* progmodes/cc-engine.el (c-state-pp-to-literal): Check for "\\s!"
as well as normal comment starter.
(c-parse-state-get-strategy): Extra return possibility
'back-and-forward.
(c-remove-stale-state-cache): Extra element CONS-SEPARATED in
return value list to indicate replacement of a brace-pair cons
with its car.
(c-parse-state-1): With 'back-and-forward, only call
c-append-lower-brace-pair-to state-cache when cons-separated.