`[[nodiscard]]` should be applied to functions where discarding the
return value is most likely a correctness issue.
- https://libcxx.llvm.org/CodingGuidelines.html
1. `hash<shared_ptr>`, `hash<unique_ptr>`, `std::integer_sequence<>`
etc.
2. Also implements fixes to
https://github.com/llvm/llvm-project/issues/169634 on the go (issues
discovered during current implementation)
---------
Co-authored-by: A. Jiang <de34@live.cn>
Co-authored-by: Hristo Hristov <zingam@outlook.com>
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