This CL moves constant to inherit from Castable.
Bug: tint:1718
Change-Id: Ifd00bcd77a838dfa560096ad95af5cf6065bfb5d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/114100
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>