Support for dynamic dimensions in 'tensor.splat' (#74626)

This feature had been marked as `TODO` in the `tensor.splat`
documentation for a while. This MR includes:

- Support for dynamically shaped tensors in the return type of
`tensor.splat` with the syntax suggested in the `TODO` comment.

- Updated op documentation.

- Bufferization support.

- Updates in op folders affected by the new feature.

- Unit tests for valid/invalid syntax, valid/invalid folding, and
lowering through bufferization.

- Additional op builders resembling those available in `tensor.empty`.