Borrow check does not detect moves in overloaded operators

This test crashes:

```
enum foo = ~uint;

impl foo: add<foo, foo> {
    pure fn add(f: foo) -> foo {
        foo(~(**self + **f))
    }
}

fn main() {
    let x = foo(~3);
    let y = x + move x;
}
```

The fix is already in my trans refactor branch, just wanted to make a note of it here.  The error is that the region hierarchy tables don't consider overloadable operators as possible scopes.
