- -Wfinal-dtor-non-final-class warns on classes with final dtors but
  not final classes.
- Error messages are better when the class is marked final.
- Fix existing issues in code base

Bug: chromium:999886
Test: no errors building
Change-Id: I62301f2f0b9108b08e0a749fbe261beb36b1996f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2575923
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>