[Attributor][FIX] Do treat byval arguments special

When we reason about the pointer argument that is byval we actually
reason about a local copy of the value passed at the call site. This was
not the case before and we wrongly introduced attributes based on the
surrounding function.

AAMemoryBehaviorArgument, AAMemoryBehaviorCallSiteArgument and
AANoCaptureCallSiteArgument are made aware of byval now. The code
to skip "subsuming positions" for reasoning follows a common pattern and
we should refactor it. A TODO was added.

Discovered by @efriedma as part of D69748.