[clang] [clang-format] Add DiagHandler for getStyle function (PR #91317)

[via cfe-commits]

pointhex wrote:

I used to redirect output. I guess that is enough. Maybe, you know a better solution for that.
```    ::testing::internal::CaptureStderr();
    auto Style = getStyle("{invalid_key=invalid_value}", "a.h", "LLVM", "", &FS, false);
    ...
    const std::string output = ::testing::internal::GetCapturedStderr();
```

https://github.com/llvm/llvm-project/pull/91317