Rollup merge of #68348 - xfix:patch-14, r=nagisa

Make iter::Empty<T> Send and Sync for any T

Continuing from #57682

It's quite funny, when I initially submitted this pull request, I said "Likely nobody will be using that property of `iter::empty`", but then a year later I got a compilation error because it wasn't `Send` and `Sync`.

Unfortunately, `PhantomData<fn() -> T>` still errors out. Oh well. I proposed `
struct PhantomFnWorkaround<T>(fn() -> T);`, but dtolnay did not like it, so using explicit implementations.

src/test/ui/threads-sendsync/sync-send-iterators-in-libcore.rs

@@ -88,6 +88,7 @@ fn main() {
     is_sync_send!((1..));
     is_sync_send!(repeat(1));
     is_sync_send!(empty::<usize>());
+    is_sync_send!(empty::<*mut i32>());
     is_sync_send!(once(1));
 
     // for option.rs