[mlir][tensor][bufferize] `tensor.empty` bufferizes to an allocation (#68080)

Make `tensor.empty` bufferizable, so that the
`-empty-tensor-to-alloc-tensor` pass becomes optional. This makes the
bufferization easier to use. `tensor.empty` used to be non-bufferizable,
so that there two separate ops, one that can be optimized away
(`tensor.empty`) and one that is guaranteed to bufferize to an
allocation (`bufferization.alloc_tensor`). With the recent improvements
of "empty tensor elimination" this is no longer needed and
`bufferization.alloc_tensor` can be phased out.