Op followed by comment should not be considered joint

```rust
for tt in "~// comment".parse::<TokenStream>().unwrap() {
    println!("{:?}", tt);
}
```

Proc-macro:

```
Op(Op { op: '~', spacing: Alone, span: Span(...) })
```

Proc-macro2:

```
Op(Op { op: '~', spacing: Joint, span: Span })
```

The comment should be considered whitespace, making the preceding op alone rather than joint.