libstdc++: Fix compatibility support in unique_ptr pretty printer

The support for the old std::unique_ptr implementation was failing,
because it tried to work on a typedef instead of the underlying type.
The test supposed to verify the support worked wasn't using a typedef,
so didn't notice the problem.

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (UniquePointerPrinter.__init__):
	Use gdb.Type.strip_typedefs().
	* testsuite/libstdc++-prettyprinters/compat.cc: Use a typedef in
	the emulated old type.