[llvm] [BOLT][NFC] Run ADRRelaxationPass in parallel (PR #67831)

[Rafael Auler via llvm-commits]

https://github.com/rafaelauler approved this pull request.

LG,  any wins in processing time from this change?

In general, we don't want to be unnecessarily running things in parallel because they significantly complicate debugging. But if we have good wins, go ahead and commit it. Also consider using a std::atomic_bool to flag it as a value that is accessed by multiple threads.

https://github.com/llvm/llvm-project/pull/67831