[libc++][ranges] iterator.concept.sizedsentinel: sized_sentinel_for and disable_sized_sentinel_for.

Based on D100160.

Reviewed By: cjdb, ldionne, Quuxplusone, #libc, miscco

Differential Revision: https://reviews.llvm.org/D100587

GitOrigin-RevId: bdd68357901d2ccdd110cf35746703ac8e46de3c
diff --git a/include/iterator b/include/iterator
index 74d04ce..918c96d 100644
--- a/include/iterator
+++ b/include/iterator
@@ -68,6 +68,13 @@
 template<class S, class I>
   concept sentinel_for = see below;                        // since C++20
 
+// [iterator.concept.sizedsentinel], concept sized_sentinel_for
+template<class S, class I>
+  inline constexpr bool disable_sized_sentinel_for = false;
+
+template<class S, class I>
+  concept sized_sentinel_for = see below;
+
 template<class Category, class T, class Distance = ptrdiff_t,
          class Pointer = T*, class Reference = T&>
 struct iterator
@@ -459,6 +466,7 @@
 #include <__iterator/iter_move.h>
 #include <__iterator/iterator_traits.h>
 #include <__iterator/readable_traits.h>
+#include <__iterator/concepts.h>
 #include <__memory/addressof.h>
 #include <__memory/pointer_traits.h>
 #include <version>