There was an existing check to ensure that `U` was an lvalue reference,
but when this check fails, overload resolution will just move right on
to the copy asignment operator, which will cause the temporary to be
assigned anyway.
Disallowing `Optional<T&>`s to be created from temporaries entirely
would be undesired, since existing code has valid reasons for creating
`Optional<T&>`s from temporaries, such as for function call arguments.
This fix explicitly deletes the `Optional::operator=(U&&)` operator,
so overload resolution stops.
75b482bbb9