C++ Mode. Fix anomaly occurring when a ">" is deleted then reinserted.

C++ Mode.  Fix anomaly occurring when a ">" is deleted then reinserted.

This fontification anomaly happened because after deleting the ">",
c-forward-<>-arglist parses the preceding identifier as a putative type but
stores it in c-found-types before it becomes clear it is not an unambiguous
type.  c-forward-<>-arglist fails, leaving the spurious type id in
c-found-types.  Fix this by "binding" c-found-types "to itself" in
c-forward-<>-arglist, and restoring the original value when that function call
fails.

* lisp/progmodes/cc-engine.el (c-copy-found-types): New function.
(c-forward-<>-arglist): Record the original value of c-found-types at the
beginning of the function, and restore it at the end on failure.

* lisp/progmodes/cc-mode.el (c-unfind-coalesced-tokens): Rewrite more
accurately.