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

[Nico Weber via cfe-commits]

nico wrote:

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

Please take a look and revert for now if it takes a while to fix.

(Maybe it's possible to rewrite the test to not need grep -Fx – none of the other tests seem to need it.)

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