[libc++] Implement P0919R3: heterogenous lookup for unordered containers

Implement heterogenous lookup for unordered containers, including the
refinement from P1690R1.

Differential Revision: https://reviews.llvm.org/D87171

GitOrigin-RevId: e5ec94a1a083e055ac2686ecc72fb4868dd7ce85
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 0f17a3a..8d7bfd4 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -206,7 +206,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_erase_if``                            ``202002L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_generic_unordered_lookup``            *unimplemented*
+    ``__cpp_lib_generic_unordered_lookup``            ``201811L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_interpolate``                         ``201902L``
     ------------------------------------------------- -----------------