PR libstdc++/59253 Improve pretty printers for smart pointers

PR libstdc++/59253 Improve pretty printers for smart pointers

	PR libstdc++/59253 (partial)
	* python/libstdcxx/v6/printers.py (SmartPtrIterator): Common iterator
	type for pointer stored by shared_ptr, weak_ptr and unique_ptr.
	(SharedPointerPrinter, UniquePointerPrinter): Treat stored values as
	children.
	* testsuite/libstdc++-prettyprinters/cxx11.cc: Update expected output
	of unique_ptr printer.
	* testsuite/libstdc++-prettyprinters/shared_ptr.cc: Update expected
	output of shared_ptr printer.

From-SVN: r256390