This removes a lot of awkward logic from the MSL writer, and means
that we now handle all module-scope variables with the same transform.
Change-Id: I782e36a4b88dafbc3f8364f7caa7f95c6ae3f5f1
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/67643
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>