C++: Add using enum upgrade and downgrade scripts.

Author: sashabu

cpp/downgrades/25e365d1e8147df0f759b604f96eb4bffea48271/old.dbscheme

@@ -0,0 +1,3 @@
+description: Revert support for using-enum declarations.
+compatibility: partial
+usings.rel: run usings.qlo

cpp/downgrades/25e365d1e8147df0f759b604f96eb4bffea48271/semmlecode.cpp.dbscheme

@@ -0,0 +1,17 @@
+class UsingEntry extends @using {
+  string toString() { none() }
+}
+
+class Element extends @element {
+  string toString() { none() }
+}
+
+class Location extends @location_default {
+  string toString() { none() }
+}
+
+from UsingEntry u, Element target, Location loc, int kind
+where
+  usings(u, target, loc, kind) and
+  kind != 3
+select u, target, loc