C++: Flow from parameters to ConstructorFieldInit

Because `ConstructorFieldInit` (member initializer lists) are not part
of the control flow graph, there was no data flow from the initial value
of parameters to their uses in member initializers. This commit adds the
necessary flow under the assumption that parameters are not overwritten
in member initializers.

cpp/ql/test/library-tests/dataflow/fields/constructors.cpp

@@ -25,8 +25,8 @@ public:
 
 void bar(Foo &f)
 {
-    sink(f.a()); // flow (through `f` and `h`) [NOT DETECTED]
-    sink(f.b()); // flow (through `g` and `h`) [NOT DETECTED]
+    sink(f.a()); // flow (through `f` and `h`)
+    sink(f.b()); // flow (through `g` and `h`)
 }
 
 void foo()