[libc++] Make `<map>` `std::multimap` constexpr as part of P3372R3#161901

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

vinay-deshmukh:vinay-issue-128661-P3372-constexpr-multimapvinay-deshmukh/llvm-project:vinay-issue-128661-P3372-constexpr-multimapCopy head branch name to clipboard