Make optional::reset noexcept, make optional::value
    work in constant expressions.
    * include/std/optional (_M_get): Make constexpr.
    (reset): Make noexcept.
    * testsuite/20_util/optional/assignment/7.cc: New.
    * testsuite/20_util/optional/observers/6.cc: New.

From-SVN: r240623