[libc++][map] Applied `[[nodiscard]]` (#169971)

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

-
https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant
- https://wg21.link/map

NOKEYCHECK=True
GitOrigin-RevId: b3bc0058321ef4ed188f99ecc0900c105e3b484b
5 files changed