This PR condenses the namespaces in the tint/diagnostic folder.
Change-Id: Id1b2afca99a8f6aeccb505682aa0f0b4b0b4bb05
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86037
Auto-Submit: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>