Isolate variables better. And add a check for out-of-date hases.
Currently if a generator is removed the hashes will stick around.

Bug: angleproject:3227
Change-Id: I9b9da245b9b29093b1f3f320a81810cd7da82395
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1500575
Commit-Queue: Jamie Madill <jmadill@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>