Implemented in both the reader and writer with E2E tests. Other
backends will ICE for now.
Bug: tint:1488
Change-Id: Ied2afa55a338347f427dee98a4076643ac432d9c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86003
Reviewed-by: Ben Clayton <bclayton@google.com>