[Attributor][FIX] Arguments of unknown functions can be undef

This should fix PR50683. The wrong assumption was that we
could always know what the callee is when we replace a call site
argument with undef. We wanted to know that to remove the `noundef`
that might be attached to the argument. Since no callee means we
did the propagation on the caller site, there is no need to remove
an attribute. It is only needed if we replace all uses and therefore
pass `undef` instead of the value that was passed in otherwise.