[libc++] [C++2b] [P1682] Add to_underlying.

* https://wg21.link/P1682

Reviewed By: ldionne, Mordante, #libc

Differential Revision: https://reviews.llvm.org/D97365

GitOrigin-RevId: 43e421417378bab378eccdf88a66a0a46304fa13
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 77010b5..693a668 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -301,5 +301,7 @@
     ``__cpp_lib_stdatomic_h``                         *unimplemented*
     ------------------------------------------------- -----------------
     ``__cpp_lib_string_contains``                     ``202011L``
+    ------------------------------------------------- -----------------
+    ``__cpp_lib_to_underlying``                       ``202102L``
     ================================================= =================