[llvm] [Support] Add cgroup-aware thread pool strategies (PR #218478)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

is there a reason not to fix up `llvm::ThreadPoolStrategy::compute_thread_count()` for all users, rather than making this opt-in? I can't imagine that LLVM would want the physical number of cores over the cgroup limit. perhaps worth an RFC

https://github.com/llvm/llvm-project/pull/218478