[llvm] [FileCheck] improve prefix validation (PR #92248)

[James Henderson via llvm-commits]

https://github.com/jh7370 requested changes to this pull request.

Please don't use force push to rebase (it's against LLVM's guidelines because it makes it harder to review what's changed since the previous version that was reviewed). Instead, use a merge commit from main, if rebasing is really necessary.

https://github.com/llvm/llvm-project/pull/92248