[llvm] [nfc] Expose `canReturn` from FunctionAttrs (PR #135650)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

### Merge activity

* **Apr 15, 4:54 PM EDT**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/135650).


https://github.com/llvm/llvm-project/pull/135650