libstdc++: Fix unique_ptr pretty printer for empty classes

libstdc++: Fix unique_ptr pretty printer for empty classes

The printer was confused when unique_ptr<T,D>::pointer is an empty
class, or the deleter is not empty. Instead of assuming the tuple has a
single _M_head_impl member manually inspect the tuple base classes to
get the first element.

	* python/libstdcxx/v6/printers.py (UniquePointerPrinter.__init__): Do
	not assume field called _M_head_impl is the first tuple element.
	* testsuite/libstdc++-prettyprinters/compat.cc: Make tuple
	implementation more accurate.
	* testsuite/libstdc++-prettyprinters/cxx11.cc: Check unique_ptr with
	empty pointer type and non-empty deleter.

(cherry picked from commit e25f488d603a6bd7570c1ffdfd9572e4b8a645de)