destructuring enum-like structs that have destructors is still allowed

#3147 "only" seems fixed for non-enum-like structs, the following code crashes right now.

```
struct S(~str);
impl Drop for S {
    fn finalize(&self) { println(**self); }
}

fn main() {
    match S(~"foo") {
        S(_s) => {}
    }
}
```
