[clang] [Clang] Fix visibility macros for c-index-test (PR #171054)

[via cfe-commits]

kkent030315 wrote:

Apparently `-DCLANG_LINK_CLANG_DYLIB=ON` is still a lot of mess right now, couldn't get it built on main.

```
RuntimeLibcallInfo.h:37:3: error: attribute 'dllexport' cannot be applied to member of 'dllexport' class
   37 |   LLVM_ABI static AnalysisKey Key;
```

https://github.com/llvm/llvm-project/pull/171054