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

[via cfe-commits]

kkent030315 wrote:

This also eliminates 5000+ unnecessary symbol exports in `clang.dll` and `clang.exe` on non-static builds.

To test this change turn `CLANG_INCLUDE_TESTS` and `LLVM_BUILD_STATIC` off. This patch should only take an effects on Windows builds.

```
cmake -S llvm -B build -G Ninja -DLLVM_ENABLE_PROJECTS="clang" -DCMAKE_BUILD_TYPE=Release -DLLVM_TARGETS_TO_BUILD="X86" -DLLVM_BUILD_TESTS=OFF -DCLANG_INCLUDE_TESTS={OFF,ON} -DLLVM_BUILD_STATIC={OFF,ON}
```

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