[libc++] Implement part of P2562R1: constexpr ranges::inplace_merge (#131947)

Drive-by changes:
- Consistently mark `std::__inplace_merge::__inplace_merge_impl`
`_LIBCPP_CONSTEXPR_SINCE_CXX26`.
- 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.
- Unblock related test coverage in constant evaluation for
`stable_partition`, `ranges::stable_sort`, `std::stable_sort`,
`std::stable_partition`, and `std::inplace_merge`.

libcxx/include/algorithm

@@ -1031,13 +1031,14 @@ namespace ranges {
   template<bidirectional_iterator I, sentinel_for<I> S, class Comp = ranges::less,
            class Proj = identity>
     requires sortable<I, Comp, Proj>
-    I inplace_merge(I first, I middle, S last, Comp comp = {}, Proj proj = {});                    // since C++20
+    constexpr I                                                                            // constexpr since C++26
+      inplace_merge(I first, I middle, S last, Comp comp = {}, Proj proj = {});            // since C++20
 
   template<bidirectional_range R, class Comp = ranges::less, class Proj = identity>
     requires sortable<iterator_t<R>, Comp, Proj>
-    borrowed_iterator_t<R>
+    constexpr borrowed_iterator_t<R>                                                       // constexpr since C++26
       inplace_merge(R&& r, iterator_t<R> middle, Comp comp = {},
-                    Proj proj = {});                                                               // since C++20
+                    Proj proj = {});                                                       // since C++20
 
   template<permutable I, sentinel_for<I> S, class Proj = identity,
            indirect_equivalence_relation<projected<I, Proj>> C = ranges::equal_to>