CppEditor: Use TypeHierarchyBuilder in FunctionHelper::overrides

...instead going the way over CppClass. This makes
FunctionHelper::overrides independent of the cppeditor plugin.

Change-Id: Ifaedb94da1f67b3876e06cce9e745aaf3c1050a7
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cpptools/typehierarchybuilder.h

@@ -53,6 +53,9 @@ public:
     CPlusPlus::Symbol *symbol() const;
     const QList<TypeHierarchy> &hierarchy() const;
 
+    bool operator==(const TypeHierarchy &other) const
+    { return _symbol == other._symbol; }
+
 private:
     CPlusPlus::Symbol *_symbol;
     QList<TypeHierarchy> _hierarchy;