C++: Fix code completion for nested classes

Fix code completion for nested classes when enclosing is
template class.
Unit tests

Task-number: QTCREATORBUG-8245 (only standalone)
Change-Id: Ib31ad4b799db927b56debd4dc3e7403404c1839d
Reviewed-by: hjk <qthjk@ovi.com>

src/libs/3rdparty/cplusplus/Templates.h

@@ -54,6 +54,8 @@ public:
 
     FullySpecifiedType &operator[](const Name *name) { return _map[name]; }
 
+    bool contains(const Name *name) const { return _map.find(name) != _map.end(); }
+
 private:
     Control *_control;
     Subst *_previous;