This CL adds const-eval for the `abs` builtin.
Bug: tint:1581
Change-Id: I6ee25c07620990f72a6962441aec62ae7665653e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/108340
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>