ICE when reporting lifetime error involving self in default methods

```
trait Thing {
    fn foo(&mut self) { (self, self); }
}

fn main() { }
```

This should fail with a lifetime error, but instead fails with

```
error: internal compiler error: Loan path LpVar(8) maps to None, not local
```

The failure occurs in `append_loan_path_to_str` while formatting the error.
