I don't object in principal, but I see no precedent for this in FileCheck.
Are you ok with this landing as is? If FileCheck should generally use report_fatal_error instead of llvm_unreachable, I feel like that should be discussed in a separate review for all occurrences.
Given your accept and the tone of your comment, I decided it's safe to land this as is. I'm fine to revert or adjust if you feel this was the wrong decision. And again, I'm open to a larger discussion about making this change throughout FileCheck.