`[[nodiscard]]` should be applied to functions where discarding the
return value is most likely a correctness issue.
- https://libcxx.llvm.org/CodingGuidelines.html#apply-nodiscard-where-relevant
The following utilities have been annotated in this patch:
- [x] `<barrier>`
- [x] `<condition_variable>`
- [x] `<latch>`
- [x] `<mutex>`
- [x] `<semaphore>`
- [x] `<thread>`
N.B. Some classes don't provide all specified methods, which were not
annotated.
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