Store in the class_typet whether the class is abstract

For interfaces and abstract classes, store in the type a value to
indicate if the class is abstract.

Author: thk123

src/java_bytecode/java_bytecode_convert_class.cpp

@@ -93,6 +93,7 @@ void java_bytecode_convert_classt::convert(const classt &c)
 
   class_type.set_tag(c.name);
   class_type.set(ID_base_name, c.name);
+  class_type.set(ID_abstract, c.is_abstract);
   if(c.is_enum)
   {
     class_type.set(

src/util/std_types.h

@@ -401,6 +401,11 @@ class class_typet:public struct_typet
 
     return false;
   }
+
+  bool is_abstract() const
+  {
+    return get_bool(ID_abstract);
+  }
 };
 
 /*! \brief Cast a generic typet to a \ref class_typet