ICE when reporting lifetime error involving self in default methods

[msullivan]

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.

[msullivan]

Part of #2794, I suppose.