C++: Fix dangling IDocument* in CppCompletionAssistProcessor

Steps to reproduce the crash:
 1. Open some long file, e.g. botan.cpp
 2. Trigger completion and close editor immediately

The IDocument pointer was only used to reference the file name. Instead
of passing an IDocument* to the IAssistInterface, pass the file name.

Change-Id: Iafce9b818806a77968a10541114bc9b7c8665f11
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: David Schulz <david.schulz@digia.com>

src/plugins/qmljseditor/qmljscompletionassist.h

@@ -120,7 +120,7 @@ class QmlJSCompletionAssistInterface : public TextEditor::DefaultAssistInterface
 public:
     QmlJSCompletionAssistInterface(QTextDocument *textDocument,
                                    int position,
-                                   Core::IDocument *document,
+                                   const QString &fileName,
                                    TextEditor::AssistReason reason,
                                    const QmlJSTools::SemanticInfo &info);
     const QmlJSTools::SemanticInfo &semanticInfo() const;