Unary move breaks linearity when moving out of enums

This code compiles and runs the destructor twice:

```
class noncopyable {
    i: (); new() { self.i = (); } drop { #error["dropped"]; }
}
enum wrapper = noncopyable;

fn main() {
    let x1 = wrapper(noncopyable());
    let x2 = move *x1;
}
```

If it's written `let x2 <- *x1`, it instead errors with `error: moving out of enum content`, which is perhaps what this should do too.
