Make optional conditionally trivially_{copy,move}_{constructible,assignable}

Make optional conditionally trivially_{copy,move}_{constructible,assignable}

* include/std/optional (_Optional_payload): Fix the comment in
the class head and turn into a primary and one specialization.
(_Optional_payload::_M_engaged): Strike the NSDMI.
(_Optional_payload<_Tp, false>::operator=(const _Optional_payload&)):
New.
(_Optional_payload<_Tp, false>::operator=(_Optional_payload&&)):
Likewise.
(_Optional_payload<_Tp, false>::_M_get): Likewise.
(_Optional_payload<_Tp, false>::_M_reset): Likewise.
(_Optional_base_impl): Likewise.
(_Optional_base): Turn into a primary and three specializations.
(optional(nullopt)): Change the base init.
* testsuite/20_util/optional/assignment/8.cc: New.
* testsuite/20_util/optional/cons/trivial.cc: Likewise.
* testsuite/20_util/optional/cons/value_neg.cc: Adjust.

From-SVN: r256694