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/cpptools/builtineditordocumentprocessor.h

@@ -53,7 +53,7 @@ public:
     void recalculateSemanticInfoDetached(bool force) override;
     void semanticRehighlight() override;
     CppTools::SemanticInfo recalculateSemanticInfo() override;
-    BaseEditorDocumentParser *parser() override;
+    BaseEditorDocumentParser::Ptr parser() override;
     CPlusPlus::Snapshot snapshot() override;
     bool isParserRunning() const override;
 
@@ -66,7 +66,7 @@ private:
     SemanticInfo::Source createSemanticInfoSource(bool force) const;
 
 private:
-    BuiltinEditorDocumentParser m_parser;
+    BuiltinEditorDocumentParser::Ptr m_parser;
     QFuture<void> m_parserFuture;
 
     CPlusPlus::Snapshot m_documentSnapshot;