[libc++][flat_multiset] Applied `[[nodiscard]]` (#169984)

`[[nodiscard]]` should be applied to functions where discarding the
return value is most likely a correctness issue.

-  https://libcxx.llvm.org/CodingGuidelines.htm
-  https://wg21.link/flat.multiset

GitOrigin-RevId: 86dc131997af87324887078221e9ac6f6d4d07e4