[Attributor] Associate the callback callee with a call site argument (if any)

If we have a callback, call site arguments were already associated with
the callback callee. Now we also associate the function with the
callback callee, thus we know ensure that the following holds true (if
all return nonnull):
   `getAssociatedArgument()->getParent() == getAssociatedFunction()`

To test this an early exit from
  `AAMemoryBehaviorCallSiteArgument::initialize``
is included as well. Without the change to getAssociatedFunction() this
kind of early exit for declarations would cause callback call site
arguments to miss out.