Make poisoned hashes SFINAE away the call operator of the hash.

Make poisoned hashes SFINAE away the call operator of the hash.

* include/bits/functional_hash.h
(__poison_hash::__enable_hash_call): New.
* include/std/optional (__optional_hash_call_base): New.
(hash<optional<_Tp>>): Derive from the new base,
move the hash function into that base.
* include/std/variant (__variant_hash_call_base_impl): New.
(__variant_hash_call_base): Likewise.
(hash<variant<_Types...>>): Derive from the new base,
move the hash function into that base.
* testsuite/20_util/optional/hash.cc: Add tests for is_callable.
* testsuite/20_util/variant/hash.cc: Likewise.

From-SVN: r244748