Keep track of the start of the argument block of a closure

Author: SarthakSingh31

src/closures.rs

@@ -335,6 +335,7 @@ pub(crate) fn rewrite_last_closure(
             ref fn_decl,
             ref body,
             fn_decl_span: _,
+            fn_arg_span: _,
         } = **closure;
         let body = match body.kind {
             ast::ExprKind::Block(ref block, _)