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

[Mike Hommey via cfe-commits]

glandium wrote:

> It looks like this breaks building at least `MultiSource` from https://github.com/llvm/llvm-test-suite/. The first failure I see is when building `llvm-test-suite/MultiSource/Applications/ClamAV/zlib_zutil.c`
> 
> ```
> In file included from /llvm-test-suite/MultiSource/Applications/ClamAV/zlib_zutil.c:10:
> In file included from test-suites/llvm-test-suite/MultiSource/Applications/ClamAV/zlib/gzguts.h:21:
> ../usr/include/stdio.h:220:7: error: expected identifier or '('
>   220 | FILE    *fdopen(int, const char *) __DARWIN_ALIAS_STARTING(__MAC_10_6, __IPHONE_2_0, __DARWIN_ALIAS(fdopen));
>       |          ^
> llvm-test-suite/MultiSource/Applications/ClamAV/zlib/zutil.h:140:33: note: expanded from macro 'fdopen'
>   140 | #        define fdopen(fd,mode) NULL /* No fdopen() */
>       |                                 ^
> llvm-project/builds/release-with-assertions/ccache-stage1/lib/clang/18/include/__stddef_null.h:26:16: note: expanded from macro 'NULL'
>    26 | #define NULL ((void*)0)
>       |                ^
> ```

These are actually part of zlib, so apart from llvm test suite having been broken by this (but was fixed), plain zlib has been broken too (although arguably, zlib is where the real issue is).

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