Back-deploy @objc actor types.
@objc actors implicitly inherit from the new, hidden
`SwiftNativeNSObject` class that inherits from `NSObject` yet provides
Swift-native reference counting, which is important for the actor
runtime's handling of zombies. However, `SwiftNativeNSObject` is only
available in the Swift runtime in newer OS versions (e.g., macOS
12.0/iOS 15.0), and is available in the back-deployed _Concurrency
library, but there is no stable place to link against for
back-deployed code. Tricky, tricky.
When back-deploying @objc actors, record `NSObject` as the superclass
in the metadata in the binary, because we cannot reference
`SwiftNativeNSObject`. Then, emit a static initializer to
dynamically look up `SwiftNativeNSObject` by name (which will find it
in either the back-deployment library, on older systems, or in the
runtime for newer systems), then swizzle that in as the superclass of
the @objc actor.
Fixes rdar://83919973.
