[clang] [clang-format] Add .clang-format.ignore for ignoring files (PR #76327)

[Björn Schäpers via cfe-commits]

HazardyKnusperkeks wrote:

In the description (and the commit messages?) you wrote `clang-format.ignore` instead of `clang-format-ignore`.

https://github.com/llvm/llvm-project/pull/76327