CC Mode: Add a workaround for syntax-ppss ignoring syntax-table prop changes

* lisp/progmodes/cc-engine.el (c-truncate-lit-pos-cache): Maintain the new
variable c-syntax-table-hwm after buffer changes.

* lisp/progmodes/cc-mode.el (c-syntax-table-hwm): New variable.
(c-before-change): Set c-syntax-table-hwm to "infinity".
(c-after-change): Call syntax-ppss-flush-cache, just before a font locking is
due to take place.
1 job for master in 3 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #2128
test-all

00:00:03

 
Name Stage Failure
failed
test-all Test There has been a runner system failure, please try again
Running with gitlab-runner 11.6.0 (f100a208)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:e1de74e67cc70422153600785c4ce4c29843a0e59e18b634c52ca133c564437f for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: failed to start shim: exec: "docker-containerd-shim": executable file not found in $PATH: unknown (executor_docker.go:1018:0s)