Add missing warnUntilSwift 6
Missed setting a diagnostic as a warning until swift 6, resulting in
noasync functions being emitted as errors in Swift 5.x. This fixes that
so they are only warnings until Swift 6.
Since `@_unavailableFromAsync` is serialized as `@available(*, noasync)`
in the swiftinterface/swiftmodule, this affects functions that are
imported from other modules as well.
