This CL adds const-eval for `cos` and `cosh`.
Bug: tint:1581
Change-Id: I8df8f979a7b351288cadccda88940fdb5a20d18f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/109561
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>