swift

6191425c - [Concurrency] Try harder to downgrade preconcurrency errors to warnings in

[Concurrency] Try harder to downgrade preconcurrency errors to warnings in Swift 5 mode. When you annotate a ValueDecl with `@preconcurrency`, the compiler should allow concurrency violations by downgrading errors in the actor isolation checker to warnings in Swift 5 mode. Previously, the actor isolation checker only checked whether the caller's context was preconcurrency when deciding to downgrade, so referencing preconcurrency declarations directly remained errors. Preconcurrency was also dropped when computing actor isolation for declarations imported from clang, which are always preconcurrency. (cherry picked from commit 57214ce7fbee9db32fda70a029b5cae60a5a8a42)