[clang] [clang-format] Fix working -assume-filename with .clang-format-ignore (PR #113100)

[via cfe-commits]

https://github.com/mydeveloperday commented:

I get what you are doing here, simply passing the input to the output even if we've said ignore the file.. Its not that I don't think this is ok I'm just wondering why the fix is here and not in the VS Code extension. I'd like to hear @owenca  opinion

https://github.com/llvm/llvm-project/pull/113100