WASHINGTON — Nevada Republican Dean Heller became the fifth GOP senator to declare his opposition to the party’s banner legislation to scuttle much of Barack Obama’s health care overhaul on Friday, more than enough to sink the measure and deliver a stinging rebuke to President Donald Trump unless some of them can be brought aboard. Echoing the other four, Heller said he opposes the measure “in this form” but does not rule out backing a version that is changed to his liking.