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;

src/plugins/cpptools/cppelementevaluator.h

@@ -81,6 +81,8 @@ private:
     QString m_diagnosis;
 };
 
+class CppClass;
+
 class CPPTOOLS_EXPORT CppElement
 {
 protected:
@@ -89,6 +91,8 @@ protected:
 public:
     virtual ~CppElement();
 
+    virtual CppClass *toCppClass();
+
     TextEditor::HelpItem::Category helpCategory;
     QStringList helpIdCandidates;
     QString helpMark;
@@ -96,7 +100,7 @@ public:
     QString tooltip;
 };
 
-class CppDeclarableElement : public CppElement
+class CPPTOOLS_EXPORT CppDeclarableElement : public CppElement
 {
 public:
     explicit CppDeclarableElement(CPlusPlus::Symbol *declaration);
@@ -109,7 +113,7 @@ public:
     QIcon icon;
 };
 
-class CppClass : public CppDeclarableElement
+class CPPTOOLS_EXPORT CppClass : public CppDeclarableElement
 {
 public:
     CppClass();
@@ -117,6 +121,8 @@ public:
 
     bool operator==(const CppClass &other);
 
+    CppClass *toCppClass() final;
+
     void lookupBases(CPlusPlus::Symbol *declaration, const CPlusPlus::LookupContext &context);
     void lookupDerived(CPlusPlus::Symbol *declaration, const CPlusPlus::Snapshot &snapshot);