Implement ' separators in C++ integer literals.
* lisp/progmodes/cc-langs.el (c-get-state-before-change-functions): insert
c-before-after-change-digit-quote into the C++ value.
(c-before-font-lock-functions): Insert c-depropertize-new-text into the values
for all languages. Insert c-before-after-change-digit-quote into the C++
value.
* lisp/progmodes/cc-mode.el (c-depropertize-CPP): Wrap the function in
c-save-buffer-state.
(c-depropertize-new-text, c-before-after-change-digit-quote): New functions.