[clang] Fix a cmake error when using the Xcode generator. (PR #119403)

[Thomas Fransham via cfe-commits]

fsfod wrote:

I guess I must of broke build for the MSVC MSBuild/vcxproj CMake generator as well, maybe it can use the same kind checks as earlier code like this
``` cmake
if(NOT XCODE AND NOT MSVC_IDE)
  target_compile_definitions("obj.${name}" PUBLIC CLANG_EXPORTS)
else()
  target_compile_definitions("${name}" PUBLIC CLANG_EXPORTS)
endif()
```

https://github.com/llvm/llvm-project/pull/119403