Four hours before Sienna Spiro is due to launch her first U.S. headlining tour, the 20-year-old singer and songwriter from London sits upstairs in the Troubadour’s empty balcony, peering down as several crew members wheel a grand piano onstage. “The fact that I’m 11-and-a-half hours from home and that this room is gonna be filled with people that have never met me and that I’ve never seen before — that’s just crazy,” she says. “I’m kind of scared.” The song that brought Spiro to West Hollywood this past Tuesday is “Die on This Hill,” a showstopping pop-soul ballad about staying in a toxic relationship — “I’ll take my pride, stand here for you,” she sings, “I’m not blind, just seeing it through” — that’s been streamed more than 300 million times on YouTube and Spotify since it came out in October. Built around tolling piano chords and Spiro’s titanic vocal, the song hit No. 9 in the U.K. and broke into the Top 20 of Billboard’s Hot 100; last month, Spiro — whose famous admirers …