[clang] [libclang] Add missing dllexport annotation (PR #147108)

[Tomohiro Kashiwada via cfe-commits]

kikairoya wrote:

> LGTM, but can you amend the PR description with the extra info you gathered, about why this hasn't been an issue in other existing configurations?

Thanks.
I've update the description.



> > It seems `AND NOT CYGWIN` should be added here.
> 
> That sounds like a good addition too, even if we're doing this as well?

Yes, it's not mandatory but seems to be recommended. I will submit a new PR later.

https://github.com/llvm/llvm-project/pull/147108