re PR libstdc++/87855 (std::optional<T> only copy-constructible if T is trivially copy-constructible)

PR libstdc++/87855

Also implement P0602R4 (variant and optional
should propagate copy/move triviality) for std::optional.
* include/std/optional (_Optional_payload): Change
the main constraints to check constructibility in
addition to assignability.
(operator=): Make constexpr.
(_M_reset): Likewise.
(_M_construct): Likewise.
(operator->): Likewise.
* testsuite/20_util/optional/assignment/8.cc: Adjust.
* testsuite/20_util/optional/assignment/9.cc: New.

From-SVN: r266278