[mlir] Add support for generating Attribute classes for ODS

[mlir] Add support for generating Attribute classes for ODS

The support for attributes closely maps that of Types (basically 1-1) given that Attributes are defined in exactly the same way as Types. All of the current ODS TypeDef classes get an Attr equivalent. The generation of the attribute classes themselves share the same generator as types.

Differential Revision: https://reviews.llvm.org/D97589