This CL moves the ast::InterpolateSampling and ast::InterpolationType to
builtin::
Change-Id: Iad9365ff629cbb7b3b03de6a4cd9355a21ce287e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/120442
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>