[libcxx][ranges] Implement indirectly_swappable.

Differential Revision: https://reviews.llvm.org/D105304

NOKEYCHECK=True
GitOrigin-RevId: edc1f0c12c836abaeeab7b0d9f7e8fb73c233ae6
diff --git a/include/iterator b/include/iterator
index b4e15c2..f6b0d2a 100644
--- a/include/iterator
+++ b/include/iterator
@@ -132,6 +132,10 @@
 template<class In, class Out>
   concept indirectly_movable_storable = see below;         // since C++20
 
+// [alg.req.ind.swap], concept indirectly_swappable
+template<class I1, class I2 = I1>
+  concept indirectly_swappable = see below;                // since C++20
+
 template<class Category, class T, class Distance = ptrdiff_t,
          class Pointer = T*, class Reference = T&>
 struct iterator                                            // deprecated in C++17