Reintroduce into_future in .await desugaring

This is a reintroduction of the remaining parts from
rust-lang#65244 that have not been relanded
yet.

Issues rust-langGH-67644, rust-langGH-67982

Author: eholk

core/src/future/into_future.rs

@@ -13,6 +13,7 @@ pub trait IntoFuture {
 
     /// Creates a future from a value.
     #[unstable(feature = "into_future", issue = "67644")]
+    #[cfg_attr(not(bootstrap), lang = "into_future")]
     fn into_future(self) -> Self::Future;
 }