Melania Trump’s lawyer, Charles J. Harder, is threatening a lawsuit over a YouTube video that suggests that Barron, the Trumps’ 10-year-old son, has autism. In a letter obtained by TMZ, Harder confirms that Barron is not autistic and portrays Rosie O-Donnell as a key perpetrator of the false claim. RELATED: Rosie O’Donnell clarifies her stance after…