C#: Add downgrade script.

Author: michaelnebel

csharp/downgrades/21ede72308c41493f19b37720d8259d5eb307f12/compiler_generated.ql

@@ -0,0 +1,7 @@
+class Modifiable extends @modifiable {
+  Modifiable() { compiler_generated(this) }
+
+  string toString() { none() }
+}
+
+select any(Modifiable m)

csharp/downgrades/21ede72308c41493f19b37720d8259d5eb307f12/expr_compiler_generated.ql

@@ -0,0 +1,7 @@
+class Expression extends @expr {
+  Expression() { compiler_generated(this) }
+
+  string toString() { none() }
+}
+
+select any(Expression e)