Reintroduce into_future in .await desugaring

This is a reintroduction of the remaining parts from
https://github.com/rust-lang/rust/pull/65244 that have not been relanded
yet.

Issues GH-67644, GH-67982

compiler/rustc_span/src/symbol.rs

@@ -196,6 +196,7 @@ symbols! {
         Implied,
         Input,
         Into,
+        IntoFuture,
         IntoIterator,
         IoRead,
         IoWrite,
@@ -734,6 +735,7 @@ symbols! {
         inout,
         instruction_set,
         intel,
+        into_future,
         into_iter,
         intra_doc_pointers,
         intrinsics,