[clang] [analyzer] Fix 'bstring.c' test on Mingw (PR #192252)

[Balázs Benics via cfe-commits]

steakhal wrote:

> Unfortunately, checking `__GNUC__` doesn't seem to work (and this does seem to break the linux CI); I believe this is because this test is run with `clang -cc1`, so many predefined defines aren't really defined at this level.
> 
> Switching this to `#if !defined(_WIN32) || defined(__MINGW32__)` does seem to work though. It's uglier unfortunately...

Got it. Thanks.

https://github.com/llvm/llvm-project/pull/192252