message: fix clang compiler warning
diff: @@ -1054,7 +1054,7 @@ end:
  for (auto &bb : f) {
  auto n = bb_num(&bb);
  - for (const auto &dst : llvm::successors(&bb)) {
  + for (auto *dst : llvm::successors(&bb)) {
  auto n_dst = bb_num(dst);
  edges[n].emplace(n_dst);
  }
  