Make coroutine-closures possible to be cloned

Author: compiler-errors

tests/ui/async-await/async-closures/without-precise-captures-we-are-powerless.stderr

@@ -56,6 +56,11 @@ LL |     outlives::<'a>(call_once(c));
 LL |
 LL |     let c = async move || { println!("{}", *x.0); };
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ move out of `x` occurs here
+   |
+help: consider cloning the value if the performance cost is acceptable
+   |
+LL |     let c = async || { println!("{}", *x.0); }.clone();
+   |                                               ++++++++
 
 error[E0597]: `c` does not live long enough
   --> $DIR/without-precise-captures-we-are-powerless.rs:33:20