C++: Bring back the StackVariable QL class

The new `StackVariable` class actually denotes what its name suggests.

cpp/ql/test/library-tests/variables/variables/variables.cpp

@@ -51,3 +51,7 @@ struct address {
   char* town;
   static char* country;
 };
+
+void hasExtern() {
+  extern int externInFunction;
+}