Span for "literal too large" points to the token after the relevant literal

``` rust
fn main() {
    let n = 999999999999999999999999999999999999

    ;
}
```

```
<anon>:4:5: 4:6 error: int literal is too large
<anon>:4     ;
             ^
```

It clearly should be pointing at the `9999...9999`.
