FXC does not support fallthrough case statements (DXC does).
Fixed: tint:1082
Change-Id: I82e1add5455e438056259f773f34bf9db05970b4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/60480
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>