[libcxx-commits] [libcxx] [libc++] Apply `[[nodiscard]]` to `wstring_convert`, `wbuffer_convert` (PR #205486)

[A. Jiang via libcxx-commits]

frederick-vs-ja wrote:

Notes:
- Previously, these were overlooked in #200726 because `wstring_convert` and `wbuffer_convert` were removed in C++26. This is also why the PR message doesn't cite sections in the latest C++ working draft.
- `<codecvt>` were also removed in C++26, but it seems that no `[[nodiscard]]` should be applied because `<codecvt>` basically provided derived classes and overriding virtual functions.

https://github.com/llvm/llvm-project/pull/205486