[mlir] Cleanup DenseArrayAttrBase definition and expose raw API

This patch cleans up the definition of `DenseArrayAttrBase` by relying
more on ODS-generated methods. It also exposes an API for using the raw
data of a dense array, similar to `DenseIntOrFPElementsAttr::getRaw`.

Reviewed By: lattner, mehdi_amini

Differential Revision: https://reviews.llvm.org/D131450