This CL moves `sem::BuiltinType` to `builtin::Function`. This makes it
available for use in the IR. The `Function` name better maps to the
usage in that it contains the builtin functions.
Bug: tint:1834
Change-Id: Ic1a26525d7845d79eb6239bdeb2b73e05f586c24
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/122606
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
