[libc++][functional] P2944R3 (partial): Comparisons for `reference_wrapper` (`reference_wrapper` operators only) (#88384)

Implements https://wg21.link/P2944R3 (partially)
Implements https://wg21.link/LWG4071 /
https://cplusplus.github.io/LWG/issue4071 (fixes build failures in the
test suite)
- https://eel.is/c++draft/refwrap.comparisons
diff --git a/libcxx/docs/FeatureTestMacroTable.rst b/libcxx/docs/FeatureTestMacroTable.rst
index 1032a9c..17b3476 100644
--- a/libcxx/docs/FeatureTestMacroTable.rst
+++ b/libcxx/docs/FeatureTestMacroTable.rst
@@ -446,7 +446,7 @@
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_rcu``                                          *unimplemented*
     ---------------------------------------------------------- -----------------
-    ``__cpp_lib_reference_wrapper``                            *unimplemented*
+    ``__cpp_lib_reference_wrapper``                            ``202403L``
     ---------------------------------------------------------- -----------------
     ``__cpp_lib_saturation_arithmetic``                        ``202311L``
     ---------------------------------------------------------- -----------------