C++: Stop flow from going through another source

Without this we get confusing results:
```
    char *userAndFile = argv[2];
    char *fileName = argv[1];
    fopen(fileName, "wb+"); // Both argv[1] and argv[2] marked as source without
                            // this change.
```

While here add some more test cases.

cpp/ql/src/Security/CWE/CWE-022/TaintedPath.ql

@@ -73,6 +73,8 @@ class TaintedPathConfiguration extends TaintTracking::Configuration {
       fileFunction.outermostWrapperFunctionCall(asSinkExpr(node), _)
     )
   }
+
+  override predicate isSanitizerIn(DataFlow::Node node) { this.isSource(node) }
 }
 
 from