Minor improvements to run_code_generation.
Minor improvements to run_code_generation.

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 And add a check for out-of-date hases.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 <7e492b4f1c8458024932de3ba475cbf015424c30@google.com>
Reviewed-by: Shahbaz Youssefi <582eeec4f08a2ccd61e3ef52115e6e7c571582de@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle