THAT WAS THE NIGHT THAT WAS . . . Yesterday at 4:10 p.m., a commercial plane touched down at O'Hare International airport and Herman and his Hermits filed off. There were flags and bands and limousines and hundreds of policement. Said little English blond rock 'n' roll star Herman - who is a high school dropout, in youthful bewilderment, "Luv, what a welcome this town gives you."     Minutes later, an air force jet also touched down on the military landing area of O'Hare and Lyndon B. Johnson, President of the United States, smiling and confident, walked off. The flags and bands and limousines and policemen were all clearly because of him.     Not much later, in McCormick Place, limousines were emptying themselves of one distinguished guest after another and from the buses came teenagers, frantic and excited. President Johnson was to give a major address in the huge upstairs dining room. Herman's Hermits were to give a concert in Arie Crown Theater on the floor below. There were to be more than 6,000 adults upstairs and approximately 5,000 youngsters downstairs.