This CL renames constant::Constant to constant::Value as it reads a bit
nicer.
Bug: tint:1718
Change-Id: I3489a271ebe229dabf98e7668bdaef4fec375534
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/114361
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>