This simplifies a lot of the data structure indexing in the program
executable class. Also renames the "DriverUniforms" and
"InternalShader" index into a single "Internal" index.

Bug: angleproject:5736
Change-Id: I2a51d8b14d5b16b438dbe636f77b11bbc045ba9a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2773321
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>