[libc++][NFC] Add 'const' to some operator()

[ldionne]

Summary

This is NFC because the function object is stateless anyway. This is
done solely for consistency with surrounding code and this was probably
an oversight in https://reviews.llvm.org/D132505.

libcxx/include/__algorithm/copy.h

struct __copy_loop {
   }
 
   template <class _InIter,
             class _OutIter,
             __enable_if_t<__has_random_access_iterator_category<_InIter>::value &&
                               !__is_segmented_iterator<_InIter>::value && __is_segmented_iterator<_OutIter>::value,
                           int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<_InIter, _OutIter>
-  operator()(_InIter __first, _InIter __last, _OutIter __result) {
+  operator()(_InIter __first, _InIter __last, _OutIter __result) const {
     using _Traits = __segmented_iterator_traits<_OutIter>;
     using _DiffT  = typename common_type<__iter_diff_t<_InIter>, __iter_diff_t<_OutIter> >::type;
 
     if (__first == __last)
       return std::make_pair(std::move(__first), std::move(__result));
 
     auto __local_first      = _Traits::__local(__result);
     auto __segment_iterator = _Traits::__segment(__result);

libcxx/include/__algorithm/copy_backward.h

struct __copy_backward_loop {
   }
 
   template <class _InIter,
             class _OutIter,
             __enable_if_t<__has_random_access_iterator_category<_InIter>::value &&
                               !__is_segmented_iterator<_InIter>::value && __is_segmented_iterator<_OutIter>::value,
                           int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<_InIter, _OutIter>
-  operator()(_InIter __first, _InIter __last, _OutIter __result) {
+  operator()(_InIter __first, _InIter __last, _OutIter __result) const {
     using _Traits           = __segmented_iterator_traits<_OutIter>;
     auto __orig_last        = __last;
     auto __segment_iterator = _Traits::__segment(__result);
 
     // When the range contains no elements, __result might not be a valid iterator
     if (__first == __last)
       return std::make_pair(__first, __result);
 

libcxx/include/__algorithm/move.h

struct __move_loop {
   }
 
   template <class _InIter,
             class _OutIter,
             __enable_if_t<__has_random_access_iterator_category<_InIter>::value &&
                               !__is_segmented_iterator<_InIter>::value && __is_segmented_iterator<_OutIter>::value,
                           int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<_InIter, _OutIter>
-  operator()(_InIter __first, _InIter __last, _OutIter __result) {
+  operator()(_InIter __first, _InIter __last, _OutIter __result) const {
     using _Traits = __segmented_iterator_traits<_OutIter>;
     using _DiffT  = typename common_type<__iter_diff_t<_InIter>, __iter_diff_t<_OutIter> >::type;
 
     if (__first == __last)
       return std::make_pair(std::move(__first), std::move(__result));
 
     auto __local_first      = _Traits::__local(__result);
     auto __segment_iterator = _Traits::__segment(__result);

libcxx/include/__algorithm/move_backward.h

struct __move_backward_loop {
   }
 
   template <class _InIter,
             class _OutIter,
             __enable_if_t<__has_random_access_iterator_category<_InIter>::value &&
                               !__is_segmented_iterator<_InIter>::value && __is_segmented_iterator<_OutIter>::value,
                           int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<_InIter, _OutIter>
-  operator()(_InIter __first, _InIter __last, _OutIter __result) {
+  operator()(_InIter __first, _InIter __last, _OutIter __result) const {
     using _Traits = __segmented_iterator_traits<_OutIter>;
     using _DiffT  = typename common_type<__iter_diff_t<_InIter>, __iter_diff_t<_OutIter> >::type;
 
     // When the range contains no elements, __result might not be a valid iterator
     if (__first == __last)
       return std::make_pair(__first, __result);
 
     auto __orig_last = __last;