swift

38cc9511 - [Concurrency] Stage in new `Async{Throwing}Stream.init(unfolding:)` errors

[Concurrency] Stage in new `Async{Throwing}Stream.init(unfolding:)` errors as warnings. Marking the closure parameter to these inits as `@Sendable` changed the inferred isolation of closure arguments in actor-isolated contexts, which caused new effects checker errors when accessing isolated properties and methods without `await`. Mark these `init`s as `@preconcurrency`, and fix the effects checker to downgrade those errors to warnings when the context of the call is `@preconcurrency`.