Make sure that the CommandInterpreter::m_command_io_handler_sp gets reset when we quit.

Currently if you run _any_ python, python has the "lldb.debugger" global variable and it has a strong reference to a lldb_private::Debugger since it is a lldb::SBDebugger object with a shared pointer.

This makes sure that your LLDB command interpreter history is saved each time you quit command line LLDB.

llvm-svn: 207164