[libc++] Implement P0339R6 (polymorphic_allocator<> as a vocabulary type)

Reviewed By: ldionne, #libc

Spies: LRFLEW, libcxx-commits, arichardson, krytarowski, jdoerfert

Differential Revision: https://reviews.llvm.org/D137739

NOKEYCHECK=True
GitOrigin-RevId: 181cce6b696e032735fb3e3d23b1db1fb9fca5cb
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 5ba070e..a137126 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -260,7 +260,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_math_constants``                      ``201907L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_polymorphic_allocator``               *unimplemented*
+    ``__cpp_lib_polymorphic_allocator``               ``201902L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_ranges``                              ``201811L``
     ------------------------------------------------- -----------------