DWARF does not mention Self type

I wrote a simple test program using the `Self` type:

```
struct Something(i32);

impl Something {
    fn x(self: &Self) -> i32 { self.0 }
}

fn main() {
    let y = Something(32);
    let z = y.x();

    ()
}
```

When I examine the resulting DWARF, I don't see any mention of `Self`.  I think it should be emitted as a typedef pointing to `Something`.
