This CL adds the `@must_use` ast node and hooks it into the
program_builder.
Bug: tint:1844
Change-Id: I2ffe99bbf29ec287446788253aa8cf63cc4da50b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/120820
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>