[llvm] [LifetimeSafety] Add PR labeler automation (PR #157820)

[Utkarsh Saxena via llvm-commits]

usx95 wrote:

### Merge activity

* **Sep 10, 9:58 AM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/157820).


https://github.com/llvm/llvm-project/pull/157820