Fix some corner cases in the recognition of C++ raw strings.

Fix some corner cases in the recognition of C++ raw strings.

These are where changing the identifier in a raw string delimiter causes
different delimiters to match eachother.

* lisp/progmodes/cc-engine.el (c-raw-string-end-delim-disrupted): New
variable.
(c-before-change-check-raw-strings): Use new variable.
(c-after-change-unmark-raw-strings): When typing into an opening delimiter or
altering its close delimiter causes the opening delimiter to match a later
closing delimiter, clear all syntax-table char properties from the opening
delimiter onwards, and set c-new-END to point max.  Also, when changing a
closing delimiter, check whether its new value matches a previously open
opening delimiter earlier in the buffer, and amend the syntax-table text
properties appropriately.