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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Oh, **a lot** of failures after 1st commit :-(

During development you can do `ninja check-llvm-filecheck` to test just `llvm/test/FileCheck`.

I typically do `ninja check-llvm check-clang check-lld`, which would give quite good coverage.

More complete coverage requires `check-all` (or `check-bolt check-clang-tools check-flang check-lldb check-mlir ...`).

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