string.IsNullOrEmpty issue: detect 'str != null && str.Length > 0' pattern
Fix false positives in FormatStringIssue when params-parameter is used in unexpanded form.
Fix false positives in SimplifyAnonymousMethodToDelegateIssue: the simplification is not possible when the lambda involves non-reference conversions.
Fix bug in AstNode.GetDescendantsImpl() that caused several unit tests to fail.