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

[via cfe-commits]

https://github.com/mydeveloperday approved this pull request.

I think this makes sense now, please wait for @owenca  and @HazardyKnusperkeks  opinion

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