[llvm] [FileCheck] forbid filecheck prefix to end with directive name (PR #92735)

[Paul T Robinson via llvm-commits]

https://github.com/pogo59 requested changes to this pull request.

My comment in the other patch was about directives read from the _file_, not command-line arguments, if that's what prompted this. Now, the check added here seems like a reasonable one, so please proceed with it, but be aware it's not what I asked for.

Because there should be no changes to the `suffixes.txt` test, there needs to be a separate test that the suffixes are rejected.

https://github.com/llvm/llvm-project/pull/92735