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/qmljseditor.cpp

@@ -1424,7 +1424,7 @@ TextEditor::IAssistInterface *QmlJSTextEditorWidget::createAssistInterface(
     if (assistKind == TextEditor::Completion) {
         return new QmlJSCompletionAssistInterface(document(),
                                                   position(),
-                                                  editor()->document(),
+                                                  editor()->document()->fileName(),
                                                   reason,
                                                   m_semanticInfo);
     } else if (assistKind == TextEditor::QuickFix) {