Output specified classical register names to QASM3

[rwakizaka]

Summary

When outputting quantum circuits in QASM3, the names of classical registers specified during circuit construction are preserved in the output.

Details and comments

We can specify a register name to add a quantum or classical register to a circuit like this:

auto qreg1 = QuantumRegister(2, std::string("q1"));
auto qreg2 = QuantumRegister(1, std::string("q2"));
auto creg1 = ClassicalRegister(2, std::string("c1"));
auto creg2 = ClassicalRegister(1, std::string("c2"));
QuantumCircuit circ(std::vector<QuantumRegister>({qreg1, qreg2}), std::vector<ClassicalRegister>({creg1, creg2}));

circ.measure(qreg2, creg2);
circ.measure(qreg1, creg1);

In the current Qiskit C++ implementation, multiple registers are combined into a single large register and their indices are adjusted, similar to the internal representation of the Qiskit C API. For example, the circuit above is output as follows:

qubit[3] q;
bit[3] c;
c[2] = measure q[2];
c[0] = measure q[0];
c[1] = measure q[1];

As a result, users have to determine which register corresponds to which index from the output, but this is obviously not intuitive. This PR resolves that issue, ensuring the QASM3 representation respects the classical register names specified by the user. For example, the circuit above will now be output as follows:

qubit[3] q;
bit[2] c1;
bit[1] c2;
c2[0] = measure q[2];
c1[0] = measure q[0];
c1[1] = measure q[1];

Note that quantum registers are combined because the indices of qubits will be mapped to physical indices by a transpiler.