[libc++] Apply `[[nodiscard]]` to `wstring_convert`, `wbuffer_convert` (#205486)
`[[nodiscard]]` should be applied to functions where discarding the
return value is most likely a correctness issue.
- https://libcxx.llvm.org/CodingGuidelines.html
- https://timsong-cpp.github.io/cppwp/n4950/depr.conversions.string
- https://timsong-cpp.github.io/cppwp/n4950/depr.conversions.buffer