This CL adds a SubstituteOverride transform which will convert
an `override` into a `const`. The transform is provided a map of
(string, double) which matches what the WebGPU API accepts as
data for overrides.
Bug: tint:1582
Change-Id: I6e6bf51b98ce4d4746f8de55128666c36735e585
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/96760
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>