message: Better fix cbackend.
diff: @@ -4399,11 +4399,7 @@ bool CWriter::visitBuiltinCall(llvm::CallInst &I, llvm::Intrinsic::ID ID,
  if (I.getParent()->getParent()->arg_empty())
  Out << "vararg_dummy_arg";
  else
  -#if ISPC_LLVM_VERSION <= ISPC_LLVM_4_0
  - writeOperand(&*(--I.getParent()->getParent()->arg_end()));
  -#else // LLVM 5.0+
  - writeOperand(&*(I.getParent()->getParent()->arg_end()-1));
  -#endif
  + writeOperand(&*(std::prev(I.getParent()->getParent()->arg_end())));
  Out << ')';
  return true;
  case llvm::Intrinsic::vaend:
  