Pretty-printer inserts an extra newline before the first match arm if it has a comment

Example:

```
fn main() {
    match 0u8 {
        // Comment
        0 => {}
        _ => {}
    }
}
```

=>

```
fn main() {
    match 0u8 {

        // Comment
        0 => {
        }
        _ => { }
    }
}
```

Affected tests:
[pretty] pretty\borrowck\borrowck-pat-enum.rs
[pretty] pretty\issue-11709.rs
[pretty] pretty\issue-28839.rs
