Concurrency: add missing Sendable requirements, conformances, and address other warnings

[DougGregor]

A few fixes to the concurrency library that address warnings:

• Add Sendable constraint to MainActor.run and a few of the back-deployment shims.
• Make _AsyncStreamCriticalStorage conform to Sendable (this is @unchecked because it's a class that handles its own locking).
• Make _runAsyncMain take an @_unsafeSendable parameter. There's a compiler bug here still, which results in a spurious warning.
• Switch the AsyncStream implementation over to withTaskCancellationHandler.
• Make the unusable property wrapper subscript for task locals unavailable.

[DougGregor]

@swift-ci please smoke test and merge