[libc++] Refactor for_each_n

[philnik777]

This does multiple things:

• if constexpr is used to simplify overload resolution
• the segmented iterator optimization is always applied, not just for random access local iterators

[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 HEAD~1 HEAD --extensions h -- libcxx/include/__algorithm/for_each_n.h libcxx/include/__algorithm/for_each_n_segment.h libcxx/include/__algorithm/ranges_for_each_n.h libcxx/include/__iterator/segmented_iterator.h libcxx/include/__iterator/unreachable_sentinel.h

View the diff from clang-format here.

diff --git a/libcxx/include/__iterator/unreachable_sentinel.h b/libcxx/include/__iterator/unreachable_sentinel.h
index 0351b88d4..543676d8a 100644
--- a/libcxx/include/__iterator/unreachable_sentinel.h
+++ b/libcxx/include/__iterator/unreachable_sentinel.h
@@ -22,7 +22,8 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-inline constexpr struct __unreachable_sentinel_t {} __unreachable_sentinel;
+inline constexpr struct __unreachable_sentinel_t {
+} __unreachable_sentinel;
 
 template <class _UnreachableSentinel,
           class _Iter,