CC Mode: correct the handling of empty strings

CC Mode: correct the handling of empty strings

In particular, have the macro cache invalidated after its last use in
c-before-change.

* lisp/progmodes/cc-langs.el (c-get-state-before-change-functions): Remove
c-invalidate-macro-cache from the entries it's in, moving it to....

* lisp/progmodes/cc-mode.el (c-before-change): Call c-invalidate-macro-cache
directly from the functions, after the call to c-invalidate-state-cache.