[ADT] Fix comment typos in STLForwardCompat.h (#157789)

st::type_identity_t is from C++20, not C++23.

Author: kazutakahirata

llvm/include/llvm/ADT/STLForwardCompat.h

@@ -36,15 +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.
+// TODO: Remove this in favor of std::type_identity<T> once we switch to C++20.
 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.
+// C++20.
 template <typename T>
 using type_identity_t // NOLINT(readability-identifier-naming)
     = typename llvm::type_identity<T>::type;