[clang] [clang-format] Honor later negated .clang-format-ignore patterns (PR #195432)

[via cfe-commits]

Dudenwatschn wrote:

Done, I believe the tests I've added should cover both re-ignore and un-ignore of files now.
Regex of CHECK10 should now properly match path-separators on windows.

https://github.com/llvm/llvm-project/pull/195432