[libc++][ranges] Implement `std::mergeable`.

Differential Revision: https://reviews.llvm.org/D119489

NOKEYCHECK=True
GitOrigin-RevId: eea3d90af181fdb66e583af53401e80ddfcc8cd1
diff --git a/include/module.modulemap b/include/module.modulemap
index a444939..ea05075 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -628,6 +628,7 @@
       module iter_swap             { private header "__iterator/iter_swap.h" }
       module iterator              { private header "__iterator/iterator.h" }
       module iterator_traits       { private header "__iterator/iterator_traits.h" }
+      module mergeable             { private header "__iterator/mergeable.h" }
       module move_iterator         { private header "__iterator/move_iterator.h" }
       module next                  { private header "__iterator/next.h" }
       module ostream_iterator      { private header "__iterator/ostream_iterator.h" }