Add rustc_on_unimplemented on Sync for cell types

Suggest using a lock instead.

tests/ui/sync/mutexguard-sync.rs

@@ -0,0 +1,13 @@
+// MutexGuard<Cell<i32>> must not be Sync, that would be unsound.
+use std::sync::Mutex;
+use std::cell::Cell;
+
+fn test_sync<T: Sync>(_t: T) {}
+
+fn main()
+{
+    let m = Mutex::new(Cell::new(0i32));
+    let guard = m.lock().unwrap();
+    test_sync(guard);
+    //~^ ERROR `Cell<i32>` cannot be shared between threads safely [E0277]
+}