This CL moves the abstract files from sem to type and updates namespaces
as needed.
Bug: tint:1718
Change-Id: I5f0be7e820fc66ea72c1ebe612a6d28034e88be6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/113341
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>