While debugging NBA2K20 MEC, fixed the following issues:
* Don't bind built-in attributes locations.
* Don't emit glGenBuffers for the default VAO.
* Include element array buffer in MEC.

Test: NBA2K20 MEC
Bug: b/160014453
Change-Id: I1b809946d67b6748b0670814be8dd090278dc38f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2307276Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Manh Nguyen <nguyenmh@google.com>
Commit-Queue: Cody Northrop <cnorthrop@google.com>