Bug: AST changed with type quantifier and inline records

```ocaml
type 'a t = A : 'a. {a:'a} -> 'a t
```

is formatted into 

```ocaml
type 'a t = A : {a: 'a} -> 'a t
```

but 
```ocaml
  type 'a t = A : 'a. 'a * 'a -> 'a t
```
works fine.

This is from the base library, as tested by the test branch, and exists at least since 0.26.0.