[libc++][ranges] Applied `[[nodiscard]]` to `repeat_view` (#173712)

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

- https://libcxx.llvm.org/CodingGuidelines.html
- https://wg21.link/range.repeat

Towards #172124

GitOrigin-RevId: 1cf4882546a0ef74952648897227bad69377daad