Implements https://github.com/gpuweb/gpuweb/pull/1447
SPIR-V Reader is still TODO, but continues to function as the offset
decoration is still supported.
Bug: tint:626
Bug: tint:629
Change-Id: Id574eb3a5c6729559382812de37b23f0c68fd406
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/43640
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Reviewed-by: David Neto <dneto@google.com>