This CL adds const-eval for the `sign` builtin.
Bug: tint:1581
Change-Id: I5d9bfd3f3f742bcba69fbb0d7f47dc57ce18e134
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/107460
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>