[llvm] llvm-shlib: Fix mingw dll exports (PR #148772)

[Keno Fischer via llvm-commits]

Keno wrote:

Yes, I did notice that there was a specific check for clang that turned `LLVM_ALWAYS_EXPORT` into `visibility("default")` rather than `__declspec(export)`. I do not know what the purpose of that check is, perhaps @fsfod can comment. It doesn't make much sense to me that it would differ between GCC and Clang, but either way I'm happy just to fix the build. I'll adjust the commit message.

https://github.com/llvm/llvm-project/pull/148772