This prepares for the addition of IR transforms. Subsequent patches
will introduce an ast::transform namespace and refactor the Transform
class into a common base class for the two types of transform.
Bug: tint:1718
Change-Id: I0088d8356341e861df68df1f4f755ee82757dd2d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/132423
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
