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

[Mircea Trofin via llvm-commits]

mtrofin wrote:

> exposing this is fine, although CFG.h doesn't seem like the right place since it's concerned with just traversing a function's CFG, not actually looking at semantics

It felt a lot, to me, like the reachability analysis above it.

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