[libcxx][ranges] Add `ranges::join_view`.

Differential Revision: https://reviews.llvm.org/D107671

NOKEYCHECK=True
GitOrigin-RevId: df324bba5c4cc0309ef4bc756fab4ebb6d67dfba
diff --git a/include/ranges b/include/ranges
index df8d419..014260a 100644
--- a/include/ranges
+++ b/include/ranges
@@ -184,6 +184,11 @@
 
   template<class W, class Bound>
     inline constexpr bool enable_borrowed_range<iota_view<W, Bound>> = true;
+
+  // [range.join], join view
+  template<input_range V>
+    requires view<V> && input_range<range_reference_t<V>>
+  class join_view;
 }
 
 */
@@ -207,6 +212,7 @@
 #include <__ranges/enable_borrowed_range.h>
 #include <__ranges/enable_view.h>
 #include <__ranges/iota_view.h>
+#include <__ranges/join_view.h>
 #include <__ranges/ref_view.h>
 #include <__ranges/reverse_view.h>
 #include <__ranges/take_view.h>