“If I Stay,” based on Gayle Forman’s popular novel for young adults, is likely to appeal to youthful fans of the book and the movie’s star, Chloë Grace Moretz, an audience that may not recognize the clichéd situations and the artless dialogue for what they are. Older moviegoers (and more discerning teenagers) are unlikely to be attracted to this unabashed tearjerker. Chloë Grace Moretz is Mia Hall, Stacy Keach is Gramps and Jakob Davies is Teddy in “If I Stay.”