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

[Tomohiro Kashiwada via cfe-commits]

kikairoya wrote:

> How so? Can you be more specific how this would break dylib builds, and what exactly is in the progress?
>
> As mentioned in the comment dllimport attribute is not strictly required, it's safe to remove it.

For code symbols, the attribute isn't strictly required, just suboptimal; lacking of `dllimport` causes additional jumps. However, for data symbols, it's mandatory (except for MinGW auto-import feature). See also: https://github.com/llvm/llvm-project/issues/149639#issuecomment-3114257062.
I'm not sure about the current progress of dylib builds for Windows.

`CLANG_ABI` is supposed to annotate `clang-cpp.dll` (which exposes C++ interfaces), but not `libclang.dll` (which exposes C interfaces). I think the problem is that `libclang.dll` is exporting C++ interfaces; that looks unintended.

The issue report uses a GNU style command line for the windows-msvc target, but such that configuration doesn't seem to have been tested well. Perhaps some of `if (MSVC)` blocks should be changed to `(WIN32 AND NOT MINGW)`.

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