LWG 2899 - Make is_move_constructible correct for unique_ptr

LWG 2899 - Make is_move_constructible correct for unique_ptr

	* include/bits/unique_ptr.h (__uniq_ptr_impl): Add move constructor,
	move assignment operator.
	(__uniq_ptr_impl::release(), __uniq_ptr_impl::reset(pointer)): Add.
	(__uniq_ptr_data): New class template with conditionally deleted
	special members.
	(unique_ptr, unique_ptr<T[], D>): Change type of data member from
	__uniq_ptr_impl<T, D> to __uniq_ptr_data<T, D>. Define move
	constructor and move assignment operator as defaulted.
	(unique_ptr::release(), unique_ptr<T[], D>::release()): Forward to
	__uniq_ptr_impl::release().
	(unique_ptr::reset(pointer), unique_ptr<T[], D>::reset<U>(U)): Forward
	to __uniq_ptr_impl::reset(pointer).
	* python/libstdcxx/v6/printers.py (UniquePointerPrinter.__init__):
	Check for new __uniq_ptr_data type.
	* testsuite/20_util/unique_ptr/dr2899.cc: New test.

From-SVN: r271158