Also includes a refactor of GLenum utils.

Bug: angleproject:5131
Change-Id: Ic2d974ef5612b3609ae66bcca087cc0442f43888
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2447042Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>