[llvm] [FileCheck] Handle directives at EOF without a trailing newline (PR #196576)

[via llvm-commits]

savchart wrote:

Thanks for the review. I’ve updated the PR. It now:

- covers the original issue with a test
- simplifies the buffer update to `Buffer = AfterSuffix;`
- adds the requested successful `CHECK-EMPTY` EOF/no-newline case
- adds a separate `CHECK:` EOF/no-pattern failure test

https://github.com/llvm/llvm-project/pull/196576