[libc++][ranges] Finish LWG issues directly related to the One Ranges Proposal.

- P1252 ("Ranges Design Cleanup") -- deprecate
  `move_iterator::operator->` starting from C++20; add range comparisons
  to the `<functional>` synopsis. This restores
  `move_iterator::operator->` that was incorrectly deleted in D117656;
  it's still defined in the latest draft, see
  http://eel.is/c++draft/depr.move.iter.elem. Note that changes to
  `*_result` types from 6.1 in the paper are no longer relevant now that
  these types are aliases;
- P2106 ("Alternative wording for GB315 and GB316") -- add a few
  `*_result` types to the synopsis in `<algorithm>` (some algorithms are
  not implemented yet and thus some of the proposal still cannot be
  marked as done);

Also mark already done issues as done (or as nothing to do):
- P2091 ("Fixing Issues With Range Access CPOs") was already implemented
  (this patch adds tests for some ill-formed cases);
- LWG 3247 ("`ranges::iter_move` should perform ADL-only lookup of
  `iter_move`") was already implemented;
- LWG 3300 ("Non-array ssize overload is underconstrained") doesn't
  affect the implementation;
- LWG 3335 ("Resolve C++20 NB comments US 273 and GB 274") was already
  implemented;
- LWG 3355 ("The memory algorithms should support move-only input
  iterators introduced by P1207") was already implemented (except for
  testing).

Differential Revision: https://reviews.llvm.org/D126053

libcxx/include/algorithm

@@ -19,12 +19,17 @@ namespace std
 {
 
 namespace ranges {
+
+  // [algorithms.results], algorithm result types
   template <class I, class F>
     struct in_fun_result;     // since C++20
 
   template <class I1, class I2>
     struct in_in_result;      // since C++20
 
+  template <class I, class O>
+    struct in_out_result;  // since C++20
+
   template <class I1, class I2, class O>
     struct in_in_out_result;  // since C++20
 
@@ -53,6 +58,9 @@ namespace ranges {
     indirect_strict_weak_order<projected<iterator_t<R>, Proj>> Comp = ranges::less>
   constexpr borrowed_iterator_t<R> ranges::max_element(R&& r, Comp comp = {}, Proj proj = {});             // since C++20
 
+  template<class I1, class I2>
+    using mismatch_result = in_in_result<I1, I2>;
+
   template <input_iterator I1, sentinel_for<_I1> S1, input_iterator I2, sentinel_for<_I2> S2,
           class Pred = ranges::equal_to, class Proj1 = identity, class Proj2 = identity>
     requires indirectly_comparable<I1, I2, Pred, Proj1, Proj2>
@@ -174,6 +182,9 @@ namespace ranges {
     constexpr range_difference_t<R>
       count_if(R&& r, Pred pred, Proj proj = {});                                   // since C++20
 
+  template<class T>
+  using minmax_result = min_max_result<T>;
+
   template<class T, class Proj = identity,
            indirect_strict_weak_order<projected<const T*, Proj>> Comp = ranges::less>
     constexpr ranges::minmax_result<const T&>
@@ -190,6 +201,9 @@ namespace ranges {
     constexpr ranges::minmax_result<range_value_t<R>>
       minmax(R&& r, Comp comp = {}, Proj proj = {});                                      // since C++20
 
+  template<class I>
+  using minmax_element_result = min_max_result<I>;
+
   template<forward_iterator I, sentinel_for<I> S, class Proj = identity,
            indirect_strict_weak_order<projected<I, Proj>> Comp = ranges::less>
     constexpr ranges::minmax_element_result<I>
@@ -201,10 +215,10 @@ namespace ranges {
       minmax_element(R&& r, Comp comp = {}, Proj proj = {});                              // since C++20
 
   template<class I, class O>
-      using copy_result = in_out_result<I, O>;                                              // since C++20
+    using copy_result = in_out_result<I, O>;                                              // since C++20
 
-    template<class I, class O>
-      using copy_n_result = in_out_result<I, O>;                                            // since C++20
+  template<class I, class O>
+    using copy_n_result = in_out_result<I, O>;                                            // since C++20
 
   template<class I, class O>
     using copy_if_result = in_out_result<I, O>;                                             // since C++20
@@ -638,19 +652,34 @@ template <class BidirectionalIterator1, class BidirectionalIterator2>
     copy_backward(BidirectionalIterator1 first, BidirectionalIterator1 last,
                   BidirectionalIterator2 result);
 
+// [alg.move], move
+template<class InputIterator, class OutputIterator>
+    constexpr OutputIterator move(InputIterator first, InputIterator last,
+                                OutputIterator result);
+
+template<class BidirectionalIterator1, class BidirectionalIterator2>
+    constexpr BidirectionalIterator2
+    move_backward(BidirectionalIterator1 first, BidirectionalIterator1 last,
+                  BidirectionalIterator2 result);
+
 template <class ForwardIterator1, class ForwardIterator2>
     constexpr ForwardIterator2    // constexpr in C++20
     swap_ranges(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2);
 
+namespace ranges {
+    template<class I1, class I2>
+    using swap_ranges_result = in_in_result<I1, I2>;
+
 template<input_iterator I1, sentinel_for<I1> S1, input_iterator I2, sentinel_for<I2> S2>
         requires indirectly_swappable<I1, I2>
     constexpr ranges::swap_ranges_result<I1, I2>
-        ranges::swap_ranges(I1 first1, S1 last1, I2 first2, S2 last2);
+        swap_ranges(I1 first1, S1 last1, I2 first2, S2 last2);
 
 template<input_range R1, input_range R2>
         requires indirectly_swappable<iterator_t<R1>, iterator_t<R2>>
     constexpr ranges::swap_ranges_result<borrowed_iterator_t<R1>, borrowed_iterator_t<R2>>
-        ranges::swap_ranges(R1&& r1, R2&& r2);
+        swap_ranges(R1&& r1, R2&& r2);
+}
 
 template <class ForwardIterator1, class ForwardIterator2>
     constexpr void                // constexpr in C++20