[clang] Fix build dllexport/dllimport issues when doing a shared build for Windows using GCC (PR #66881)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

Thanks, I wasn't aware of this issue (I don't routinely try building with `-DBUILD_SHARED_LIBS=ON`, which I presume is what you've done to trigger this). See 592e935e115ffb451eb9b782376711dab6558fe0 for earlier context on this issue; therefore I'd prefer to fix this as I do in #71393; can you confirm if that change works for you as well?

https://github.com/llvm/llvm-project/pull/66881