Tracy Chapman is a national treasure, and if that’s a fact you’ve doubted in the 20-plus years since “Fast Car,†her latest performance might challenge you to reconsider. Chapman, making a rare television appearance, performed a heart-stopping rendition of Ben E. King’s classic “Stand by Me†Thursday night as a special send-off to David Letterman, who says the song was a childhood favorite. Excuse me while I shed an infinite stream of tears.