Borrow check needs to consider moves into patterns

```
struct Dummy {
    x: int
}

impl Dummy {
    fn new(x: int) -> Dummy {
        Dummy { x: x }
    }
}

impl Drop for Dummy {
    fn drop(&self) {
        error!("Destructor %d", self.x);
    }
}

fn welp<T>(a: ~T) -> T {
    let ~x = a;
    x
}

fn main() {
    let o = ~Dummy::new(5);
    welp(o);
}
```

(edit: a similar problem appears for `let a@b = c`; see comment below.)
