Merge #9038

9038: Folding range for return types r=Veykril a=MozarellaMan

For issue #8957 

![return type fold](https://user-images.githubusercontent.com/48062697/119979082-5c62e100-bfb2-11eb-9729-1dea1ce74de1.gif)


Co-authored-by: Ayomide Bamidele <48062697+MozarellaMan@users.noreply.github.com>

crates/rust-analyzer/src/to_proto.rs

@@ -534,6 +534,7 @@ pub(crate) fn folding_range(
         | FoldKind::Consts
         | FoldKind::Statics
         | FoldKind::WhereClause
+        | FoldKind::ReturnType
         | FoldKind::Array => None,
     };