[llvm] Introduce LDBG_OS() macro as a variant of LDBG() (PR #157194)

[Andrzej Warzyński via llvm-commits]

https://github.com/banach-space commented:

I really like how you refactored `LDBG_OS` not to require `LLVM_DEBUG` and to be more consistent with `LDBG` 🙏🏻 

This change is actually quite dense and not something I contribute to, so it took me a while to parse. Mostly make sense - I've left some minor suggestion. I will do another pass once these are addressed.

Thanks! In particular the unit tests will be a great source of documentation.

https://github.com/llvm/llvm-project/pull/157194