Complex sentence: So popular was the song "Young at Heart", that the film was titled after the song, having no title until the song's success led it to be used not only for the title, but also for music over the opening and closing credits.
Split into simpler sentences: So popular was the song "Young at Heart" that the film was also titled "Young at Heart", having had no title until the song's success. The song's popularity led it to be used not only for the title, but also for music over the opening and closing credits.