They already exist in a `ast::type` namespace, so `ast::type::BlahType` is just stuttering.
This is more important now that Is<> and As<> use the full type name.
Change-Id: I7c661fe58cdc33ba7e9a95c82c996a799786661f
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/34321
Reviewed-by: dan sinclair <dsinclair@chromium.org>