[TF:XLA] Improve/refactor the handling of resource types/shapes.

Previously we used an xla::Shape to track the shape of a resource (Variable, TensorArray, Stack) shape. The xla::Shape described how the resource was represented to XLA, e.g., as a (buffer, size) pair for a Stack resource.

Instead, separate the TensorFlow abstract shape representation from the XLA shape representation and track it separately. This leads to simpler and more readable code.

PiperOrigin-RevId: 184310694