C++: drop uniqueness of id in derived types

Author: IdrissRio

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -720,7 +720,7 @@ case @derivedtype.kind of
 ;
 
 derivedtypes(
-    unique int id: @derivedtype,
+    int id: @derivedtype,
     string name: string ref,
     int kind: int ref,
     int type_id: @type ref