PR libstdc++/90520
	* python/libstdcxx/v6/printers.py (UniquePointerPrinter.__init__):
	Raise exception if unique_ptr tuple member has unknown structure.
	* python/libstdcxx/v6/xmethods.py (UniquePtrGetWorker.__call__):
	Adjust worker to support new __uniq_ptr_data base class. Do not
	assume field called _M_head_impl is the first tuple element.

From-SVN: r271363