Bug: comment placement on "_" part of record patterns

Hi,

Consider the following piece of code where a comment is relative to the `; _` part of a pattern:

```ocaml
type t = {x: int; y: int}

let to_string {x ; _ (* we should print y *)} =
  string_of_int x
```

The pattern gets formatted as `{x (* we should print y *); _}`.

(Note that this is low priority - I stumbled upon this one recently)