Add TODO

Author: kparzysz

llvm/include/llvm/ADT/STLForwardCompat.h

@@ -36,12 +36,15 @@ template <typename T>
 using remove_cvref_t // NOLINT(readability-identifier-naming)
     = typename llvm::remove_cvref<T>::type;
 
+// TODO: Remove this in favor of std::type_identity<T> once we switch to C++23.
 template <typename T>
 struct type_identity // NOLINT(readability-identifier-naming)
 {
   using type = T;
 };
 
+// TODO: Remove this in favor of std::type_identity_t<T> once we switch to
+// C++23.
 template <typename T>
 using type_identity_t // NOLINT(readability-identifier-naming)
     = typename llvm::type_identity<T>::type;