[libc++][ranges] Applied `[[nodiscard]]` to `drop_while_view` (#173211)

`[[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/ranges
- https://wg21.link/range.drop.while

Towards #172124

NOKEYCHECK=True
GitOrigin-RevId: b988d413245c8580eec77da705e24cb3730a4e12