Update dangling pointer tests

Author: gavincrawford

core/src/cell.rs

@@ -2273,6 +2273,7 @@ impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<UnsafeCell<U>> for UnsafeCell<T>
 /// See [`UnsafeCell`] for details.
 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
 #[repr(transparent)]
+#[rustc_diagnostic_item = "sync_unsafe_cell"]
 #[rustc_pub_transparent]
 pub struct SyncUnsafeCell<T: ?Sized> {
     value: UnsafeCell<T>,