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

[Zixu Wang via cfe-commits]

zixu-w 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 :(

Yeah... Not entirely familiar with the zlib development and contribution process, but looks like most of the PRs were just closed and changes committed separately some time after (presumably with a release?).

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