swift

1eacf039 - [sil-opt] Tighten up how we handle strict concurrency and upcoming features so we match swift-frontend.

[sil-opt] Tighten up how we handle strict concurrency and upcoming features so we match swift-frontend. Specifically: 1. I added the "upcoming feature already enabled in this swift-version" diagnostic. 2. I taught sil-opt that in swift-6 mode strict concurrency is enabled by default.