This CL adds the basis of the Metal Shading Language backend.
Bug: tint:8
Change-Id: I85976250eb41ac12203a5db116444e993c3d09d4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/23700
Reviewed-by: David Neto <dneto@google.com>
Use the shortcut for zero-values: a type constructor without
any parameters. Scalars still use plain literals like false, 1u, 1.0.
Change-Id: Ie436f1af28cbab0b4c87a07b057deb04632eb534
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/23680
Reviewed-by: dan sinclair <dsinclair@google.com>