Clang: Highlight invalid declarations as text

...instead of the corresponding declaration color.

Task-number: QTCREATORBUG-18686
Change-Id: Ice4d84816351af79efa286f49516c392bd80da86
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangbackend/source/cursor.h

@@ -61,6 +61,7 @@ public:
     bool isStaticMethod() const;
     bool isCompoundType() const;
     bool isDeclaration() const;
+    bool isInvalidDeclaration() const;
     bool isLocalVariable() const;
     bool isReference() const;
     bool isExpression() const;