CC Mode: Add accurate handling for backslash in C line and block comments

CC Mode: Add accurate handling for backslash in C line and block comments

This is needed to handle the idiosyncratic meaning of backslash in comments in
the C and C++ standards.

* lisp/progmodes/cc-engine.el: Correct a spelling error.

* lisp/progmodes/cc-mode.el (c-before-change-fix-comment-escapes)
(c-after-change-fix-comment-escapes): New functions.

* lisp/progmodes/cc-langs.el (c-get-state-before-change-functions): Add
c-before-change-fix-comment-escapes to the C/Objc and C++ values.
(c-before-font-lock-functions): Add c-after-change-fix-comment-escapes to the
C/Objc and C++ values.