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

[Arthur Eubanks via llvm-commits]

https://github.com/aeubanks approved this pull request.

I was getting confused with the CFG.h in IR vs Analysis. Analysis/CFG.h still currently doesn't look at semantics and only looks at the CFG proper, but I think it's fine to extend it to look at semantics

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