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

[Keno Fischer via llvm-commits]

Keno wrote:

> This was implemented on the linker side in both LLD and binutils ld.bfd, and on the compiler side in Clang - but I didn't try to implement it in GCC.

Yes, I was aware of that, although I'm still missing why that implies that `LLVM_ALWAYS_EXPORT` can't be `__declspec(export)` in this case (while still using hidden visibility to add the extra drectve). Can you fill in the missing link for me?

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