Switch from #[expect] to #[allow] because of a clippy bug

Author: antoyo

src/back/lto.rs

@@ -633,7 +633,8 @@ pub fn optimize_thin_module(
             save_temp_bitcode(cgcx, &module, "thin-lto-after-pm");
         }
     }*/
-    #[expect(clippy::let_and_return)]
+    // FIXME: switch to #[expect] when the clippy bug is fixed.
+    #[allow(clippy::let_and_return)]
     module
 }