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.
1 job for master in 74 minutes and 55 seconds (queued for 4 seconds)
Status Job ID Name Coverage
failed #4398


Name Stage Failure
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
make[1]: *** [check-expensive] Error 2
Makefile:294: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1