castable.h is bigger than it needs to be, and pretty much every tint .cc file includes castable.h
Reduce the amount of code that .cc files that don't use Switch() need to compile.
Change-Id: Ibb4e8b0bc7104ad33a7f2f39587c7d9e749fee97
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/123401
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
