The shader translator outputs arbitrary location indices for fragment
outputs.  Once compiled by glslang, the SPIR-V transformer modifies
these decorations.

Bug: angleproject:3394
Change-Id: Ib9d8336bccc392e789e4d93031fdcce9c466b7a6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2011214
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>