[libc++][expected] Applied `[[nodiscard]]` (#170245)
[[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/expected.bad.void
- https://wg21.link/expected.bad
- https://wg21.link/expected.expected
- https://wg21.link/expected.void
- https://wg21.link/expected.unexpected
It was already discussed not to mark the type `std::expected` as
`[[nodiscard]]` see:
https://github.com/llvm/llvm-project/pull/139651
https://github.com/llvm/llvm-project/pull/130820
Also:
https://github.com/llvm/llvm-project/pull/154943
