This Cl adds the code to emit scalar and type constructors from the
Metal backend.
Bug: tint:8
Change-Id: I95c713568ae3a73b82f8c9e10119e29e3469893e
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/23707
Reviewed-by: David Neto <dneto@google.com>
dan sinclair