[libc++][unordered_multimap] Applied `[[nodiscard]]` (#171659)

`[[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/unord.multimap

NOKEYCHECK=True
GitOrigin-RevId: a5b7c42ab2deaa7c1c8426d1b8f360e561bc1acb