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

[Tomohiro Kashiwada via cfe-commits]

kikairoya wrote:

I believe this breaks dylib builds (`-DCLANG_LINK_CLANG_DYLIB=ON`), which is still in progress.
`__declspec(dllimport)` is essential for that.

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