[clang] Add a flag to preserve the old macro behaviour. (PR #174895)

[Snehasish Kumar via cfe-commits]

snehasish wrote:

> Probably inculde in the commit description and/or in comments in the code that this is intended to be short-lived (perhaps give a rough time estimate, or a cutoff date where we think this flag's usefulness should've expired?)

Filed #175249 and added a TODO in a comment to cleanup. It's safe to assume it's good to cleanup by EOY 2026 though I expect it should be gone earlier.

https://github.com/llvm/llvm-project/pull/174895