PR libstdc++/66017
	* include/bits/stl_tree.h (_Rb_tree_node): Use __aligned_membuf.
	(_Rb_tree_iterator, _Rb_tree_const_iterator): Support construction
	from _Base_ptr.
	(_Rb_tree_const_iterator::_M_const_cast): Remove static_cast.
	(_Rb_tree::begin, _Rb_tree::end): Remove static_cast.
	* include/ext/aligned_buffer.h (__aligned_membuf): New type using
	alignment of _Tp as a member subobject, not as a complete object.
	* python/libstdcxx/v6/printers.py (StdRbtreeIteratorPrinter): Lookup
	_Link_type manually as it might not be in the debug info.

From-SVN: r223745