rustfmt removes oddly placed comments

rustfmt likes to remove commented lines that are found in odd places.  This appears to affect both stable and nightly `rustfmt`.

### Input File
```rust
struct X<T>
// where
//     T: Clone
{
    inner: String,
}
```

### After rustfmt
```rust
struct X<T> {
    inner: String,
}
```