PR libstdc++/87704 fix unique_ptr(nullptr_t) constructors

Using a delegating constructor to implement these constructors means
that they instantiate the destructor, which requires the element_type to
be complete. In C++11 and C++14 they were specified to be delegating,
but that was changed as part of LWG 2801 so in C++17 they don't require
a complete type (as was intended all along).

	PR libstdc++/87704
	* include/bits/unique_ptr.h (unique_ptr::unique_ptr(nullptr_t)): Do
	not delegate to default constructor.
	(unique_ptr<T[], D>::unique_ptr(nullptr_t)): Likewise.
	* testsuite/20_util/unique_ptr/cons/incomplete.cc: New test.

From-SVN: r265423