[libc++][multiset] Applied `[[nodiscard]]` (#171654)

`[[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/multiset

NOKEYCHECK=True
GitOrigin-RevId: 3e2a8e2effd19cc6271dd64578868696625bbe79
2 files changed