This PR condenses the namespaces in the tint/ast folder.
Change-Id: I77a84fb03f921b4db7135572005a08563f2fb60b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86038
Reviewed-by: Ben Clayton <bclayton@google.com>
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>