[clang] [libclang][Cygwin] Use __declspec(dllexport) for libclang on Cygwin (PR #147122)

[Martin Storsjö via cfe-commits]

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

LGTM, thanks.

But we should wait for https://github.com/llvm/llvm-project/pull/147108 to land first before merging this.

https://github.com/llvm/llvm-project/pull/147122