Drive-by: Consistently mark __inplace_merge::__inplace_merge_impl

This function template is only called by other functions that becomes
constexpr since C++26, and it itself calls `std::__inplace_merge` that
is constexpr since C++26. So it's better to mark it
`_LIBCPP_CONSTEXPR_SINCE_CXX26`.

Author: frederick-vs-ja

libcxx/include/__algorithm/ranges_inplace_merge.h

@@ -41,7 +41,7 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 namespace ranges {
 struct __inplace_merge {
   template <class _Iter, class _Sent, class _Comp, class _Proj>
-  _LIBCPP_HIDE_FROM_ABI static constexpr auto
+  _LIBCPP_HIDE_FROM_ABI static _LIBCPP_CONSTEXPR_SINCE_CXX26 auto
   __inplace_merge_impl(_Iter __first, _Iter __middle, _Sent __last, _Comp&& __comp, _Proj&& __proj) {
     auto __last_iter = ranges::next(__middle, __last);
     std::__inplace_merge<_RangeAlgPolicy>(