This should save on a few string comparisons. This CL also adds a
helper function that converts from an entry point enum ID to a string
entry point name.

Bug: angleproject:3611
Change-Id: I47434235ca25203ef1d6814897d81afab444a7fd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1719065
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>