[libc++] Mark __cpp_lib_constexpr_memory as being implemented

GitOrigin-RevId: 5d0d465ad4d25398519b85879d61fc126abd641c
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 7028ff0..997d213 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -210,7 +210,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_iterator``                  *unimplemented*
     ------------------------------------------------- -----------------
-    ``__cpp_lib_constexpr_memory``                    *unimplemented*
+    ``__cpp_lib_constexpr_memory``                    ``201811L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_numeric``                   ``201911L``
     ------------------------------------------------- -----------------