std::thread::detach() rarely crashes in Parallel.cpp

[Rafael Avila de Espindola via llvm-commits]

NAKAMURA Takumi <geek4civic at gmail.com> writes:

> Rafael, I tried a few stories you suggested.
>
> 1) Launch noop(return nullptr) and detach it in main context
> I saw crash in pthread_detach.
>
> 2) Avoid grandchildren. Just launch threads in main context.
> I couldn't happen crash in a million run.
> Note, it's a devil's proof.

Care to benchmark that and send for review?

Cheers,
Rafael