CPP: Add dataflow FP with output arguments

cpp/ql/test/library-tests/dataflow/dataflow-tests/flowOut.cpp

@@ -0,0 +1,20 @@
+int source();
+void sink(int);
+
+void source_ref(int *toTaint) { // $ ir-def=*toTaint ast-def=toTaint
+    *toTaint = source();
+}
+
+
+
+void modify_copy(int* ptr) { // $ ast-def=ptr
+  int deref = *ptr;
+  int* other = &deref;
+  source_ref(other);
+}
+
+void test_output() {
+   int x = 0;
+   modify_copy(&x);
+   sink(x); // $ ir
+}