Fix async desugaring providing wrong input to procedural macros.
Fixes#60674.
This PR fixes a minor oversight introduced by #60535 where unused `mut` binding modes were removed from the arguments to an `async fn` (as they were added to the statement that we insert into the closure body). However, this meant that the input to procedural macros was incorrect. This removes that and instead fixes the `unused_mut` error that it avoided.
r? @cramertj
cc @taiki-e
45b09453db