Improve validation readablity by introducing a bit field class
and by changing how the CL error code is returned.

Bug: angleproject:6001
Change-Id: I51deb745454e5281de725481edef85eb30be28c7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2919690Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: John Plate <jplate@google.com>