Migrate `unused-revision-names` check from tidy to compiletest

[jieyouxu]

Eventually. compiletest has the revision info. Currently //@ unused-revision-names is recognized by compiletest but handled by tidy.

[jieyouxu]

(E-hard because it needs some untangling to make it not a mess)

[nilotpal-n7]

@rustbot claim

[jieyouxu]

Note:

Getting directive handling code out of its current situation is a much higher priority. I don't want to introduce complications for cleanup efforts on that front in the short/medium term.