Add default TR::CodeGenerator constructor

This is needed to prepare for the JitBuilder and TestCompiler CodeGenerators
being removed.

Signed-off-by: Daryl Maier <maier@ca.ibm.com>

compiler/codegen/CodeGenerator.hpp

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2000, 2018 IBM Corp. and others
+ * Copyright (c) 2000, 2020 IBM Corp. and others
  *
  * This program and the accompanying materials are made available under
  * the terms of the Eclipse Public License 2.0 which accompanies this
@@ -39,6 +39,10 @@ class OMR_EXTENSIBLE CodeGenerator : public OMR::CodeGeneratorConnector
     */
    CodeGenerator(TR::Compilation *comp) :
       OMR::CodeGeneratorConnector() {}
+
+   CodeGenerator() :
+      OMR::CodeGeneratorConnector() {}
+
    };
 }