[libc++] Merge the segmented iterator code for {copy,move}_backward

[philnik777]

This removes a bit of code duplication and might simplify future segmented iterator optimitations.

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff origin/main HEAD --extensions h -- libcxx/include/__algorithm/copy_backward.h libcxx/include/__algorithm/for_each_segment.h libcxx/include/__algorithm/move_backward.h --diff_from_common_commit

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View the diff from clang-format here.

diff --git a/libcxx/include/__algorithm/for_each_segment.h b/libcxx/include/__algorithm/for_each_segment.h
index de0ed243d..3445f34c2 100644
--- a/libcxx/include/__algorithm/for_each_segment.h
+++ b/libcxx/include/__algorithm/for_each_segment.h
@@ -54,7 +54,7 @@ __for_each_segment_backward(_SegmentedIterator __first, _SegmentedIterator __las
   using _Traits = __segmented_iterator_traits<_SegmentedIterator>;
 
   auto __sfirst = _Traits::__segment(__first);
-  auto __slast = _Traits::__segment(__last);
+  auto __slast  = _Traits::__segment(__last);
 
   // We are in a single segment, so we might not be at the beginning or end
   if (__sfirst == __slast) {