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>