Ban non-void return from __construct, __destruct, __clone?

I've found one case of this in Facebook:

```
class Foo {
public function __clone(): Foo {
return new Foo( ... );
}
}
```

This looks like a misunderstanding about how __clone works, as it's called after a copy, instead of to actually do the copy. See http://php.net/manual/en/language.oop5.cloning.php

t5158846 for fixing the issue in Facebook.