This change breaks the assumption everywhere that varyings can be
identified uniquely by name throughout all stages of the pipeline.

It further implements linking of varyings by location, if specified.

Bug: angleproject:4355
Change-Id: Ie45e48879008c3f0c22d1da3d0d26f37c655e54e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2030026
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>