[libcxx] [llvm] [libc++] Fix optional<T>::transform to optional<T&> (GH#220332) (PR #220500)

[A. Jiang via llvm-commits]

================
@@ -321,6 +321,18 @@ constexpr bool test_ref() {
     assert(&(*o2) == &j);
   }
 
+  // https://github.com/llvm/llvm-project/issues/220332
----------------
frederick-vs-ja wrote:

```suggestion
  // https://llvm.org/PR220332
  // Ensure `optional::transform` to properly produce `optional<T&>` despite privateness of data member.
```

https://github.com/llvm/llvm-project/pull/220500