[clang] [clang][PP] Add extension to predefine target OS macros (PR #74676)

[Mike Hommey via cfe-commits]

glandium wrote:

> We have provided a fix for zlib ([madler/zlib#895](https://github.com/madler/zlib/pull/895)) which was accepted by @madler.

But nowhere released, not even in the repository :(

https://github.com/llvm/llvm-project/pull/74676