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

[Tomohiro Kashiwada via cfe-commits]

kikairoya wrote:

> > Still it may be good to figure out why this hasn't been an issue so far, for whoever otherwise were using these dllexport annotations.
> 
> I suspect it relates to module definition file (*.def). On MinGW, the symbol is properly exported without annotation.

Regular win32 targets use `libclang.def` to choose symbols to be exported (generated from `libclang-generic.exports` through `LLVM_EXPORT_SYMBOL_FILE`) but it's disabled on most of Unix platforms.
https://github.com/llvm/llvm-project/blob/53359252688692f2b0e25f529335848db94cc166/clang/tools/libclang/CMakeLists.txt#L95-L98

It seems `AND NOT CYGWIN` should be added here.

Even if exported symbols are controlled by `libclang.def`, annotating `__declspec(dllimport)` correctly is still preferred.


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