The shader translator outputs arbitrary location indices.  Once compiled
by glslang, the SPIR-V transformer modifies these decorations.  If the
transform feedback extension is used, it will also add the relevant
decorations to the varyings that are captured.

Bug: angleproject:3394
Change-Id: I5ecafd0536408612a5d4b920dbabbfabe650657c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2008468
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>