[libc++] Implement P2273R3 (`constexpr` `unique_ptr`)

Reviewed By: mordante, #libc

Differential Revision: https://reviews.llvm.org/D131315

NOKEYCHECK=True
GitOrigin-RevId: 30dadaa2eb499395f5bef44b9e1a18db6360ad1f
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 8860578..8d38beb 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -312,6 +312,8 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_cmath``                     *unimplemented*
     ------------------------------------------------- -----------------
+    ``__cpp_lib_constexpr_memory``                    ``202202L``
+    ------------------------------------------------- -----------------
     ``__cpp_lib_constexpr_typeinfo``                  *unimplemented*
     ------------------------------------------------- -----------------
     ``__cpp_lib_forward_like``                        ``202207L``