[mlir][ODS] Support specialized Attribute class for Enums

Add a feature to `EnumAttr` definition to generate
specialized Attribute class for the particular enumeration.

This class will inherit `StringAttr` or `IntegerAttr` and
will override `classof` and `getValue` methods.

With this class the enumeration predicate can be checked with simple
RTTI calls (`isa`, `dyn_cast`) and it will return the typed enumeration
directly instead of raw string/integer.

Based on the following discussion:
https://llvm.discourse.group/t/rfc-add-enum-attribute-decorator-class/2252

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D97836