[Attributor] fix AANoUndef initialization

Currently, `AANoUndefImpl::initialize` mistakenly always indicates optimistic fixpoint for function returned position.
 This is because an associated value is `Function` in the case, and `isGuaranteedNotToBeUndefOrPoison` returns true for Function.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D86361