[libc++] Implement P0627R6 (Function to mark unreachable code)

Reviewed By: ldionne, Quuxplusone, #libc

Spies: arichardson, mstorsjo, libcxx-commits, mgorny

Differential Revision: https://reviews.llvm.org/D119152

NOKEYCHECK=True
GitOrigin-RevId: 2a8f9a5e95dec3adcdee603358dc03def6d2245e
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 0991609..8877ec0 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -350,6 +350,6 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_to_underlying``                       ``202102L``
     ------------------------------------------------- -----------------
-    ``__cpp_lib_unreachable``                         *unimplemented*
+    ``__cpp_lib_unreachable``                         ``202202L``
     ================================================= =================