[llvm] [IR] Allow nofree on return values (PR #203491)

[Ralf Jung via llvm-commits]

================

----------------
RalfJung wrote:

Due to how this was incoherent for a long time, would it make sense to update the `dereferenceable` attribute docs to explicitly say that this means "dereferenceable at the moment the argument/return value is passed (but may be freed later at any point)"?

https://github.com/llvm/llvm-project/pull/203491