Report whether one was generated so that Dawn knows to use the
`-fpreserve-invariance` compiler option.
Bug: tint:772
Change-Id: Ife1eb05265646727dc864f12f983781af4df3777
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/57644
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>