Remove the ability to directly construct a DenseElementsAttr with a raw character...

Remove the ability to directly construct a DenseElementsAttr with a raw character buffer. This made assumptions about how DenseElementsAttr structured its internal storage, which may change in the future. To replace the existing use cases, a few utility methods have been added:

* 'get' methods that allow constructing from an ArrayRef of integer or floating point values.
* A 'reshape' method to allow for changing the shape without changing the underlying data.

PiperOrigin-RevId: 252067898