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

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

AFAIK the dllexport annotations have been added using some automated tools, so it may be good to find the person who added the other annotations, so it can be looked into why this was missing here, if the annotations otherwise were seemingly complete enough.

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