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

[David Blaikie via cfe-commits]

dwblaikie 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?)

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