[libc++] Implement part of P2562R1: constexpr `std::stable_partition` (#128868)
Drive-by changes:
- Enables no-memory case for Clang.
- Enables `robust_re_difference_type.compile.pass.cpp` and
`robust_against_proxy_iterators_lifetime_bugs.pass.cpp` test coverage
for `std::stable_sort` in constant evaluation since C++26. The changes
were missing in the PR making `std::stable_sort` `constexpr`.
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 7b4cb8e..0c7cea1 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -1498,7 +1498,7 @@
Predicate pred);
template <class ForwardIterator, class Predicate>
- ForwardIterator
+ constexpr ForwardIterator // constexpr in C++26
stable_partition(ForwardIterator first, ForwardIterator last, Predicate pred);
template<class ForwardIterator, class Predicate>
