CC Mode. Fix fontification bug with unterminated quotes on adjacent lines

CC Mode.  Fix fontification bug with unterminated quotes on adjacent lines

In particular, with these unterminated quotes on each of two adjacent lines,
the following text was spuriously fontified with string face.

* lisp/progmodes/cc-defs.el
(c-search-backward-char-property-with-value-on-char): New macro.

* lisp/progmodes/cc-mode.el (c-clear-string-fences): Check whether there is an
unmatched quote at a lower buffer position which should match the current
quote, rather than wrongly assuming the latter is unmatched and marking it
with a punctuation syntax.
(c-font-lock-fontify-region): Ensure all pertinent parts of the buffer have
string fence properties applied before performing any syntactic operations on
it; in particular, this applies to a quote at an earlier buffer position which
"matches" one inside the region about to be fontified.