CppEditor: make convert from/to pointer aware of auto

Change-Id: Ic378ea1f7fc58fdb9351c9eaa9e9602e568d5ac9
Task-number: QTCREATORBUG-13605
Reviewed-by: Robert Loehning <robert.loehning@theqtcompany.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>