Fix C++ type hierarchy

Dynamic casts between library boundaries tend to fail. Add an "explicit
cast" to CppClass via virtual functions as a quickfix.
This is a recurring issue, e.g. d2769f3003, 3f11ef9216 and
2ffd0e2d0d to name just a few...

This was introduced in 5e861d2be6

Task-number: QTCREATORBUG-20001
Change-Id: Ie5a89a028d587e4e9d1ecec920a7c7d17497dbde
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/cpptools/cppelementevaluator.cpp

@@ -63,6 +63,11 @@ CppElement::CppElement() : helpCategory(TextEditor::HelpItem::Unknown)
 CppElement::~CppElement()
 {}
 
+CppClass *CppElement::toCppClass()
+{
+    return nullptr;
+}
+
 class Unknown : public CppElement
 {
 public:
@@ -156,6 +161,11 @@ bool CppClass::operator==(const CppClass &other)
     return this->declaration == other.declaration;
 }
 
+CppClass *CppClass::toCppClass()
+{
+    return this;
+}
+
 void CppClass::lookupBases(Symbol *declaration, const LookupContext &context)
 {
     typedef QPair<ClassOrNamespace *, CppClass *> Data;