They are only used to mark angle_BaseVertex and angle_BaseInstance
uniforms, but they make it complicated for the rest of the code to
identify them as uniforms.

Bug: angleproject:4889
Change-Id: I0c632bb595dc9a7fcda19db11ecb27539b1c33e6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2818144Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>