Provide `#[diagnostic::on_move(message)]` for smart pointers to explain why they aren't `Copy` and what to do instead

[estebank]

Given a situation like #149861 where we emit

error[E0382]: use of moved value: `results`
  --> src/main.rs:30:22
   |
22 |     let results = Arc::new(Mutex::new(Vec::new()));
   |         ------- move occurs because `results` has type `Arc<std::sync::Mutex<Vec<i32>>>`, which does not implement the `Copy` trait
...
28 |     for i in 0..20 {
   |     -------------- inside of this loop
29 |         // let results = Arc::clone(&results);
30 |         pool.execute(move || {
   |                      ^^^^^^^ value moved into closure here, in previous iteration of loop
31 |             let mut r = results.lock().unwrap();
   |                         ------- use occurs due to use in closure
...

we should have a way of annotating Arc to explain that even though it isn't Copy, you can cheaply clone it.