Move-assign a basic_waitable_timer from another.

basic_waitable_timer & operator=(
    basic_waitable_timer && other);
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template<
    typename Executor1>
enable_if< is_convertible< Executor1, Executor >::value, basic_waitable_timer & >::type operator=(
    basic_waitable_timer< Clock, WaitTraits, Executor1 > && other);
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