C++: add upgrade and downgrade scripts

Author: IdrissRio

cpp/downgrades/3c45f8b9e71ec723bf50c40581e1f18f4f25e290/lambdas.ql

@@ -0,0 +1,7 @@
+class LambdaExpr extends @lambdaexpr {
+  string toString() { none() }
+}
+
+from LambdaExpr lambda, string default_capture, boolean has_explicit_return_type
+where lambdas(lambda, default_capture, has_explicit_return_type, _)
+select lambda, default_capture, has_explicit_return_type