[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/include/module.modulemap b/include/module.modulemap
index 0ef457a..95ff1ec 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -961,6 +961,7 @@
       module swap                { private header "__utility/swap.h" }
       module to_underlying       { private header "__utility/to_underlying.h" }
       module transaction         { private header "__utility/transaction.h" }
+      module unreachable         { private header "__utility/unreachable.h" }
     }
   }
   module valarray {