#[derive(Clone)] always require generic arguments to be Clone, which is too restrictive

# [derive(Clone)] should not require generic arguments to be Clone if it's not necessary.

```
use std::rc::Rc;

struct S;

#[derive(Clone)]
struct Works(Rc<S>);

#[derive(Clone)]
struct Fails<T>(Rc<T>);

fn main() {
    let w = Works(Rc::new(S));
    w.clone(); // ok

    let f = Fails(Rc::new(S));
    f.clone(); // fails
}
```
