Function get_syntax_file_and_diagnostics 

pub fn get_syntax_file_and_diagnostics<'a>(
    db: &'a SimpleParserDatabase,
    file_id: FileId<'a>,
) -> (SyntaxFile<'a>, Diagnostics<'a, ParserDiagnostic<'a>>)

Returns the syntax_file and diagnostic of a file in the db.