HBO’s “Girls,” premiering Sunday at 10:30 p.m., is nominally a comedy, a sort of angsty, indie, younger bizarro take on “Sex and the City,” as we tag along in the lives of four very different twentysomething female friends. But there is also an element of horror at work as the bad choices, humiliations, and meltdowns made by or visited upon each member of the quartet leads to cringeworthy moments that reverberate with heartbreaking realism.