[libcxx][ranges] Add `unreachable_sentinel`.

Differential Revision: https://reviews.llvm.org/D107920

NOKEYCHECK=True
GitOrigin-RevId: 4ac87e33785eaaed8a41d4880075ff346fab8a0d
diff --git a/include/module.modulemap b/include/module.modulemap
index c50a140..f7c899c 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -548,6 +548,7 @@
       module reverse_access        { private header "__iterator/reverse_access.h" }
       module reverse_iterator      { private header "__iterator/reverse_iterator.h" }
       module size                  { private header "__iterator/size.h" }
+      module unreachable_sentinel  { private header "__iterator/unreachable_sentinel.h" }
       module wrap_iter             { private header "__iterator/wrap_iter.h" }
     }
   }