[clang] [clang-format] Optimize processing .clang-format-ignore files (PR #76733)

[Owen Pan via cfe-commits]

owenca wrote:

> This still breaks tests on win: http://45.33.8.238/win/88113/step_7.txt

I didn't get any email for the failure. Is this a regular LLVM buildbot?

If it chokes on `grep -Fx`, can you (or whoever has access to it) install a POSIX-compliant grep? It seems all other Windows buildbots got no problems.

https://github.com/llvm/llvm-project/pull/76733