Fix TODOs dating back to when types were an AST / SEM hybrid concept.
Bring the `arch.md` to reflect how things work today.
Bug: tint:724
Change-Id: I6bf4174158cf490f2839aeed78164b66e3410f27
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/96141
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>