This can be used to convert them to strings for debugging. It can also
be helpful for frame capture and replay. Currently the enums are output
as GLenum values.

Bug: angleproject:3611
Change-Id: Ifcd04449e0ef61e125e0db65aa0dcc2bf48b38ca
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1678399Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>