Support `std::vector<measure_handle>::operator=` in kernels

[khalatepradnya]

Summary

Enable the QEC vectorized cross-round detector patterns shown below. They are the vectorized counterparts to the scalar prev_s0 = s0; idiom from the internal spec, and they do not compile today.

Examples to enable

C++ — vectorized cross-round, local-only (loop)

#include <cudaq.h>
#include <vector>
__qpu__ void rep_code_vectorized(int nRounds) {
  cudaq::qvector ancillas(2);
  auto prev = mz(ancillas);
  for (int r = 1; r < nRounds; r++) {
    auto curr = mz(ancillas);
    cudaq::detectors(prev, curr);
    prev = curr;                    // carry current round forward
  }
}

C++ — vectorized cross-round with reference parameter

#include <cudaq.h>
#include <vector>
__qpu__ void surface_code_round(cudaq::qvector<>& data,
                                cudaq::qvector<>& ancillas,
                                std::vector<cudaq::measure_result>& prev) {
  auto syndromes = mz(ancillas);
  cudaq::detectors(prev, syndromes);
  prev = syndromes;                 // update caller's `prev` for next round
}

Current Behavior

Compilation fails

$ cudaq-quake rep_code_vectorized.cpp
error: ../cudaq/lib/Frontend/nvqpp/ConvertExpr.cpp:1595: not yet implemented: unhandled std::vector member function, operator=
LLVM ERROR: fatal error, aborting.

Expected Behavior

Program compiles and runs successfully.

Prior work

#4573 attempted to fix by using shallow copy semantics, but, was generating wrong IR.