QmlJS: Fix infinite loop when scanning for exported C++ types.

Setting extra diagnostics would call updateDocument which would in turn
trigger another scan.

Change-Id: I3810a380cdf716a12767d94ff82dc30f8ae3954d
Reviewed-on: http://codereview.qt.nokia.com/3917
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>
Reviewed-by: Leandro T. C. Melo <leandro.melo@nokia.com>

src/libs/cplusplus/CppDocument.h

@@ -177,6 +177,9 @@ public:
         QString text() const
         { return _text; }
 
+        bool operator==(const DiagnosticMessage &other) const;
+        bool operator!=(const DiagnosticMessage &other) const;
+
     private:
         int _level;
         QString _fileName;