internal: remove one more immutable tree

crates/syntax/src/ast.rs

@@ -47,6 +47,12 @@ pub trait AstNode {
     {
         Self::cast(self.syntax().clone_for_update()).unwrap()
     }
+    fn clone_subtree(&self) -> Self
+    where
+        Self: Sized,
+    {
+        Self::cast(self.syntax().clone_subtree()).unwrap()
+    }
 }
 
 /// Like `AstNode`, but wraps tokens rather than interior nodes.