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

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo approved this pull request.

LGTM, thanks!

It's worth pointing out (and adding to the commit message, through the PR description) that building the DLL wasn't broken if building with Clang instead of GCC. (I do test that build configuration nightly.) If building with Clang, some of the attribute macros end up expanding to setting default/hidden visibility, which was why they didn't end up as dllexports.

So perhaps amend the message, "is broken in LLVM 20+, if building with GCC. (If building with Clang for mingw, the symbol visibility macros currently set default/hidden visibility as opposed to dllexport, so that build did work all along.)" - or something like that?

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