What should we add?

Currently in the control-flow builders, there is no simple way to cause the block to contain a qubit that idles for the duration of the block. There is not always a major reason to want the additional data dependency on an idle qubit, but this becomes more useful for the upcoming box, where one might want to manually specify the grouping of certain idle qubits for the purposes of configuring error mitigation.

We can add a QuantumCircuit.noop method that accepts any Qubit specifier and considers them to be "used" by the current active block. It is a standard CircuitError to noop a qubit that is not in the circuit, as it is for regular operations.

edit: medium priority because it's wanted by internal teams for demos, but not a 100% blocker.