[libc++][C++26] `constexpr std::stable_sort` (part of P2562)#105379

PaulXiCao wants to merge 2 commits intollvm:mainllvm/llvm-project:mainfrom

PaulXiCao:constexpr_stable_sortPaulXiCao/llvm-project:constexpr_stable_sortCopy head branch name to clipboard