[libcxxabi] [llvm] [ItaniumDemangle][NFC] Rename argument to not conflict with SS from ptrace_abi.h (PR #179079)

[Louis Dionne via llvm-commits]

https://github.com/ldionne requested changes to this pull request.

I very strongly think that you need to fix (by removing) the `#define` in those system headers instead. This code does not need to be guarded against arbitrary user defines since it lives in the built library. We can't start to accommodate arbitrary unhygienic defines in headers all around. For all I know, under that reasoning, `SSub` might be defined by another platform and so this change would be breaking for that platform.

So, while I agree that this change doesn't cause any harm, I am pushing back against it on principle because it's not the right way to address these issues, and creating churn in the project doesn't move us forward.

https://github.com/llvm/llvm-project/pull/179079