[Attributor] Provide convenient helpers for isAssumedRead{None,Only}

We have two attributes that can answer readnone queries. While there is
a dependence between them, it seems best to not force the users to know
what AA to ask. The helpers also allow to check for readonly nicely.

Test changes show where we now deduce readnone but haven't before,
mostly because we only asked AAMemoryBehavior and not AAMemoryLocation.
AANoAlias has not been ported to the new API yet.