CppTypeHierarchy: Don't create/store QIcon in non-GUI thread

Store Utils::CodeModelIcon::Type enum instead.
Drop unneeded CPlusPlus::Symbol *declaration from CppDeclarableElement.
Remove unused CppClass::operator==().

Task-number: QTCREATORBUG-28529
Change-Id: Ie3487fa5f73d714ef375c42f1e64023ccc4194ee
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/cppeditor/cppelementevaluator.cpp

@@ -94,8 +94,7 @@ public:
 // CppDeclarableElement
 CppDeclarableElement::CppDeclarableElement(Symbol *declaration)
     : CppElement()
-    , declaration(declaration)
-    , icon(Icons::iconForSymbol(declaration))
+    , iconType(Icons::iconTypeForSymbol(declaration))
 {
     Overview overview;
     overview.showArgumentNames = true;
@@ -135,11 +134,6 @@ CppClass::CppClass(Symbol *declaration) : CppDeclarableElement(declaration)
     tooltip = qualifiedName;
 }
 
-bool CppClass::operator==(const CppClass &other)
-{
-    return this->declaration == other.declaration;
-}
-
 CppClass *CppClass::toCppClass()
 {
     return this;