[llvm] [lit] Optionally exclude xfail tests (PR #151191)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

### Merge activity

* **Jul 31, 7:48 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/151191).


https://github.com/llvm/llvm-project/pull/151191