Add __gnu_test::NullablePointer utility to testsuite

Add __gnu_test::NullablePointer utility to testsuite

	* testsuite/20_util/allocator_traits/members/allocate_hint_nonpod.cc:
	Use operator-> to access raw pointer member.
	* testsuite/23_containers/vector/59829.cc: Likewise.
	* testsuite/23_containers/vector/bool/80893.cc: Likewise.
	* testsuite/libstdc++-prettyprinters/cxx11.cc: Use NullablePointer.
	* testsuite/util/testsuite_allocator.h (NullablePointer): New utility
	for tests.
	(PointerBase, PointerBase_void): Derive from NullablePointer and use
	its constructors and equality operators. Change converting
	constructors to use operator-> to access private member of the other
	pointer type.
	(PointerBase_void::operator->()): Add, for access to private member.
	(operator-(PointerBase, PointerBase)): Change to hidden friend.
	(operator==(PointerBase, PointerBase)): Remove.
	(operator!=(PointerBase, PointerBase)): Remove.

From-SVN: r271160