C++: Fix crash after triggering completion and closing editor

Fix use-after-free for the following case:
  1. Open an editor
  2. Trigger a long processing completion
     (e.g. simulate with QThread::msleep in
      CppCompletionAssistInterface::getCppSpecifics)
  3. ...and immediately close the editor (e.g. with Ctrl+W)
  4. Wait until it crashes.

The completion thread relied on the BuiltinEditorDocumentParser object,
which is deleted once the editor is closed. Fixed by sharing the
ownership of that object between the *EditorDocumentProcessor and the
completion assist interface.

This case came up when doing tests for the bug report below.

Task-number: QTCREATORBUG-14991
Change-Id: I0b009229e68fc6b7838740858cdc41a32403fe6f
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>

src/plugins/texteditor/codeassist/codeassistant.cpp

@@ -89,7 +89,6 @@ public:
     virtual bool eventFilter(QObject *o, QEvent *e);
 
 private:
-    void finalizeRequest();
     void proposalComputed();
     void processProposalItem(AssistProposalItem *proposalItem);
     void handlePrefixExpansion(const QString &newPrefix);
@@ -251,7 +250,7 @@ void CodeAssistantPrivate::requestProposal(AssistReason reason,
         connect(m_requestRunner, &ProcessorRunner::finished,
                 this, &CodeAssistantPrivate::proposalComputed);
         connect(m_requestRunner, &ProcessorRunner::finished,
-                this, &CodeAssistantPrivate::finalizeRequest);
+                m_requestRunner, &QObject::deleteLater);
         connect(m_requestRunner, &ProcessorRunner::finished,
                 q, &CodeAssistant::finished);
         assistInterface->prepareForAsyncUse();
@@ -383,12 +382,6 @@ void CodeAssistantPrivate::handlePrefixExpansion(const QString &newPrefix)
     notifyChange();
 }
 
-void CodeAssistantPrivate::finalizeRequest()
-{
-    if (ProcessorRunner *runner = qobject_cast<ProcessorRunner *>(sender()))
-        delete runner;
-}
-
 void CodeAssistantPrivate::finalizeProposal()
 {
     stopAutomaticProposalTimer();