[mlir][ods] Added EnumAttr, an AttrDef implementation of enum attributes

`EnumAttr` is a pure TableGen implementation of enum attributes using `AttrDef`. This is meant as a drop-in replacement for `StrEnumAttr`, which is soon to be deprecated. `StrEnumAttr` is often used over `IntEnumAttr` because its more readable in MLIR assembly formats. However, storing and manipulating strings is not efficient. Defining `StrEnumAttr` can also be awkward and relies on a lot of special logic in `EnumsGen`, and has some hidden sharp edges.

Also, `EnumAttr` stores the enum directly,  removing the need to convert to/from integers when calling attribute getters on ops.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D115181