Don't keep around references to old documents

The code completion was keeping around references to old documents after
the completion finished. This caused documents to stay in memory when
unloading projects, up until the next time you used the completion.

src/plugins/cpptools/cppcodecompletion.cpp

@@ -1027,6 +1027,10 @@ bool CppCodeCompletion::partiallyComplete(const QList<TextEditor::CompletionItem
 void CppCodeCompletion::cleanup()
 {
     m_completions.clear();
+
+    // Set empty map in order to avoid referencing old versions of the documents
+    // until the next completion
+    typeOfExpression.setDocuments(QMap<QString, Document::Ptr>());
 }
 
 int CppCodeCompletion::findStartOfName(const TextEditor::ITextEditor *editor)