[tf.data] Fix C++ shape inference for `Dataset.concatenate()`.

Previously, we were returning an unknown shape in
`Dataset::output_shapes()` for the "most specific compatible shape"
between the two inputs. While this does not cause correctness problems
(since the unknown shape *is* compatible), we gain the ability to
raise errors earlier when more shape information is available.

PiperOrigin-RevId: 215764530