It is now official. President Reagan ha signed the bill, and the United States has its first honorary citizen since Winston Churchill. He is Raoul Wallenberg, honored for saving the lives of 100,000 Hungarians, most of them Jews, when he was a member of neutral Sweden's legation in Budapest during World War II. Is it too much to ask that the Soviet Union, one of the allies against Nazism, respond to the world's compassionate concern about Mr. Wallenberg? The Soviets arrested him as a suspected spy in 1945 and have refused to reopen the case snce asserting that he perished behind bars two years later. Now Mr. Reagan joins those who cite evidence that he is still alive in a Soviet prison. The Kremlin could honor its own people's wartime suffering and heroism by addressing the evidence and clearing the air.