Make BCO# lifted
In #17424 (closed) Simon PJ noted that there is a potentially unsafe occurrence
of unsafeCoerce#, coercing from an unlifted to lifted type. However,
nowhere in the compiler do we assume that a BCO# is not a thunk.
Moreover, in the case of a CAF the result returned by createBCO
will
be a thunk (as noted in [Updatable CAF BCOs]). Consequently it seems
better to rather make BCO# a lifted type.