CC Mode. Allow fontification of "wrong" style comments with warning face.

This fixes bug #4192.

* etc/NEWS: Add a new entry.

* lisp/progmodes/cc-defs.el (c-font-lock-flush): New macro.

* lisp/progmodes/cc-cmds.el (c-toggle-comment-style): On toggling the comment
style, invoke c-font-lock-flush when c-mark-wrong-style-of-comment is non-nil,
to cause that marking to be done instead on the other style of comment.

* lisp/progmodes/cc-fonts.el (c-maybe-font-lock-wrong-style-comments): New
function.
(c-cpp-matchers): Call c-maybe-font-lock-wrong-style-comments when
appropriate.

* lisp/progmodes/cc-vars.el (c-mark-wrong-style-of-comment): New customizable
option.

* doc/misc/cc-mode.texi (top level, Indentation Commands, Guessing the Style,
Custom Macros): For some opening quote marks, correct '' to ``.
(Minor Modes): Add an xref to the new page "Wrong Comment Style" in a
footnote.
(Wrong Comment Style): New page.
1 job for master in 25 minutes and 26 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #4158
test-all

00:25:26

 
Name Stage Failure
failed
test-all Test
make[2]: *** [cus-load.el] Error 255
Makefile:152: recipe for target 'cus-load.el' failed
make[2]: Leaving directory '/builds/emacs/emacs/lisp'
Makefile:411: recipe for target 'lisp' failed
make[1]: *** [lisp] Error 2
make[1]: Leaving directory '/builds/emacs/emacs'
Makefile:1117: recipe for target 'bootstrap' failed
make: *** [bootstrap] Error 2
ERROR: Job failed: exit code 1