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

[Owen Pan via cfe-commits]

owenca wrote:

It had passed my local tests (x64 Native Tools Command Prompt for VS 2022) and the buildkite win build, so I wonder if it has something to do with the grep in the failed buildbot (only https://lab.llvm.org/buildbot/#/builders/123/builds/23808 so far) as I only used the POSIX options -F and -x. Should I turn it off for Windows like we did to [style-on-command-line.cpp](https://github.com/llvm/llvm-project/blob/8c72ff716b3e4b298695fa3faf6add860c6dbcb2/clang/test/Format/style-on-command-line.cpp#L50)?

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