Produce a warning if the attribute is missing for integral vertex
outputs or fragment inputs. This will become an error in the future,
as per the WGSL spec.
Add the attribute to E2E tests.
Bug: tint:1224
Change-Id: Ia1f353f36cb7db516cf9e8b4877423dec3b3e711
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/67160
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>