C++: Make 'follow symbol' work with forward declared classes.

If it encounters a forward declaration, it tries to find the class
declaration globally now.

Task-number: QTCREATORBUG-20

src/libs/cplusplus/CppDocument.h

@@ -372,6 +372,7 @@ public:
                                      const QString &fileName) const;
 
     Symbol *findMatchingDefinition(Symbol *symbol) const;
+    Class *findMatchingClassDeclaration(Symbol *symbol) const;
 
 private:
     void simplified_helper(Document::Ptr doc, Snapshot *snapshot) const;