This is only valid on structure members and entry point return
types. Additionally, this can only be applied to a position builtin.
Bug: tint:772
Change-Id: Iffd774ca768ab66373678ecf0eb57c766767e92b
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/57641
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>