The `fallthrough` statement is being removed from WGSL. This CL adds
a deprecation warning into the current implementation to prepare folks
for the upcoming removal.
Bug: tint:1644
Change-Id: I599984b6a30b39312c4b794a9ecd70f0f626c759
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/98841
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
