Fix bugs in the c-parse-state mechanism. Reuse some markers instead of

Fix bugs in the c-parse-state mechanism.  Reuse some markers instead of
continually generating new ones.

cc-engine.el (c-state-old-cpp-beg-marker, c-state-old-cpp-end-marker):
New variables.
(c-append-lower-brace-pair-to-state-cache): Start a backward search for
"}" definitively outside CPP constructs.
(c-remove-stale-state-cache): Inform the caller of a need to search back
for a brace pair in certain circumstances.
(c-state-maybe-marker): New macro.
(c-parse-state): Reuse markers when appropriate.