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.
Marking a stored property as unavailable with `@available` should be banned,
just like it already is banned for potential unavailability. Unavailable code
is not supposed be reachable at runtime, but unavailable properties with
storage create a back door into executing arbitrary unavailable code at
runtime:
```
@available(*, unavailable)
struct Unavailable {
init() {
print("Unavailable.init()")
}
}
struct S {
@available(*, unavailable)
var x = Unavailable()
}
_ = S() // prints "Unavailable.init()"
```
Resolves rdar://107449845
