Account for async closures when pointing at lifetime in return type

[estebank]

error: lifetime may not live long enough
  --> $DIR/higher-ranked-return.rs:11:46
   |
LL |           let x = async move |x: &str| -> &str {
   |  ________________________________-________-____^
   | |                                |        |
   | |                                |        let's call the lifetime of this reference `'2`
   | |                                let's call the lifetime of this reference `'1`
LL | |             x
LL | |         };
   | |_________^ returning this value requires that `'1` must outlive `'2`

instead of

error: lifetime may not live long enough
  --> $DIR/higher-ranked-return.rs:11:46
   |
LL |           let x = async move |x: &str| -> &str {
   |  ________________________________-________----_^
   | |                                |        |
   | |                                |        return type of async closure `{async closure body@$DIR/higher-ranked-return.rs:11:46: 13:10}` contains a lifetime `'2`
   | |                                let's call the lifetime of this reference `'1`
LL | |             x
LL | |         };
   | |_________^ returning this value requires that `'1` must outlive `'2`

[rustbot]

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