[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.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D85610