[clang] [Clang][Cygwin] attempt to fix building shared libclang. (PR #138351)

[Mateusz Mikuła via cfe-commits]

mati865 wrote:

I think it works when you build with Clang rather than GCC. Clang will not export symbols with hidden visibility.

https://github.com/llvm/llvm-project/pull/138351