Implement 2801, Default-constructibility of unique_ptr.

Implement 2801, Default-constructibility of unique_ptr.

* include/bits/unique_ptr.h (__uniq_ptr_impl::_DeleterConstraint): New.
(unique_ptr::_DeleterConstraint): Likewise.
(unique_ptr()): Constrain.
(unique_ptr(pointer)): Likewise.
(unique_ptr(nullptr_t)): Likewise.
(unique_ptr<_Tp[], _Dp>::_DeleterConstraint): New.
(unique_ptr<_Tp[], _Dp>::unique_ptr()): Constrain.
(unique_ptr<_Tp[], _Dp>::unique_ptr(_Up)): Likewise.
(unique_ptr<_Tp[], _Dp>::unique_ptr(nullptr_t)): Likewise.
* testsuite/20_util/unique_ptr/assign/48635_neg.cc: Adjust.
* testsuite/20_util/unique_ptr/cons/cv_qual_neg.cc: Likewise.
* testsuite/20_util/unique_ptr/cons/default.cc: New.
* testsuite/20_util/unique_ptr/cons/ptr_deleter_neg.cc: Adjust.

From-SVN: r244054