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

[Tomohiro Kashiwada via cfe-commits]

kikairoya wrote:

https://github.com/llvm/llvm-project/blob/53359252688692f2b0e25f529335848db94cc166/clang/tools/libclang/CMakeLists.txt#L88-L93

MSVC doesn't use `libclang.def` and doesn't export `clang_install_aborting_llvm_fatal_error_handler`.
```
$ llvm-readobj --coff-exports '/c/Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/Llvm/bin/libclang.dll' | grep clang_install || echo not found
not found
```

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