Implement P0513R0, Poisoning the Hash.

Implement P0513R0, Poisoning the Hash.

* include/bits/functional_hash.h (__poison_hash): New.
* include/bits/unique_ptr.h
(hash<unique_ptr<_Tp, _Dp>>): Derive from __poison_hash.
* include/std/optional (hash<optional<_Tp>>): Likewise.
* include/std/variant (hash<variant<_Types...>>): Likewise.
* testsuite/20_util/default_delete/48631_neg.cc: Adjust.
* testsuite/20_util/default_delete/void_neg.cc: Likewise.
* testsuite/20_util/optional/hash.cc: New.
* testsuite/20_util/unique_ptr/assign/48635_neg.cc: Adjust.
* testsuite/20_util/unique_ptr/cons/cv_qual_neg.cc: Adjust.
* testsuite/20_util/unique_ptr/hash/1.cc: Add tests for
poisoned fancy pointer hashes.
* testsuite/20_util/variant/hash.cc: New.

From-SVN: r242402