libstdc++: Avoid forming T* in unique_ptr(auto_ptr<U>&&) constraints [PR116529]

PR 116529 shows that std::unique_ptr<X&, D> is currently unusable
because the constructor taking std::auto_ptr (which is a non-standard
extension since C++17) tries to form the invalid type X&* during
overload resolution. We can use the `pointer` type in the constructor
constraints, instead of trying to form an invalid type. The
std::auto_ptr constructor can never actually match for the case where
element_type is a reference, so we just need it to produce a
substitution failure instead of being ill-formed.

LWG 4144 might make std::unique_ptr<X&, D> ill-formed, which would
invalidate this new test. We would have to remove this test in that
case. Using `pointer` in the constructor from std::auto_ptr would not be
needed to support the std::unique_ptr<X&, D> case, but would not cause
any harm either.

libstdc++-v3/ChangeLog:

	PR libstdc++/116529
	* include/bits/unique_ptr.h (unique_ptr(auto_ptr<U>&&)):
	Use pointer instead of T*.
	* testsuite/20_util/unique_ptr/creation/116529.cc: New test.

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