The attribute makes the declaration unavailable from the perspective of clients
of the module's public interface and was creating a loophole that admitted
inappropriate unavailability.
10.50 was once greater than any real macOS version, but now it compares
less than real released versions, which makes these tests depend on the
deployment target unnecessarily. Update these tests to use even larger
numbers to hopefully keep them independent a little longer.
Destructors are always called if declared, so allowing deinit to be declared as
unavailable (or potentially unavailable) creates a type checking loophole that
allows unavailable code to execute at runtime:
```
class C {
@available(*, unavailable)
deinit {
print("Oops")
}
}
_ = C() // prints "Oops"
```
Resolves rdar://106409012 and https://github.com/apple/swift/issues/63854.
