negate_unsigned feature gate triggered for user-defined Neg

``` rust
struct S;
impl std::ops::Neg for S {
    type Output = u32;
    fn neg(self) -> u32 { 0 }
}
fn main() { -S; }
```

```
<anon>:6:13: 6:15 error: unary negation of unsigned integers may be removed in the future
<anon>:6 fn main() { -S; }
                     ^~
```
