Span note for last semicolon points at one space after the last expression rather than at the semicolon

This function

``` rust
fn blah() -> int {
    1  ;
}
```

yields the following messages:

```
test2.rs:2:6: 2:6 note: consider removing this semicolon:
test2.rs:2     1  ;
                ^
test2.rs:1:1: 3:2 error: not all control paths return a value
test2.rs:1 fn blah() -> int {
test2.rs:2     1  ;
test2.rs:3 }
error: aborting due to previous error
```
