Closing over an outer function arg is not caught by capture pass

[marijnh]

This currently crashes the compiler (Map.get miss in ty_param_count_and_ty_for_def in typeck.rs when looking up outer_arg):

fn foo(int outer_arg) {
    fn local() -> int {
        ret outer_arg;
    }
}

[marijnh]

This is handled by 58ec5d1