[Attributor][NFC][AAPotentialValues] Change interface of PotentialValuesState
Previously `PotentialValuesState` inherited `BooleanState`. We have to add `getAssumed` to the state in order to use `clampStateAndIndicateChange` (which will be used in `AAPotentialValuesArgument`). However `BooleanState::getAssumed` is not a virtual function and we cannot override it. Therefore, I changed the state not to inherit `BooleanState` and add `getAssumed` to it.