[mlir][ODS] Add `ConstantEnumCase` (#78992)

Specifying an enum case of an enum attr currently requires the use of
either `NativeCodeCall` or a `ConstantAttr` specifying the full C++ name
of the enum case. The disadvantages of both are less readable code due
to including C++ expressions and very few checks of any kind, creating
C++ code that does not compile instead.

This PR adds `ConstantEnumCase`, a kind of `ConstantAttr` which
automatically derives the correct value representation from a given enum
and the string representation of an enum case. It supports both
`EnumAttrInfo`s (enums wrapping `IntegerAttr`) and `EnumAttr` (proper
dialect attributes). It even supports bit-enums, allowing one to list
multiple enum cases and have them be combined. If an enum case is not
found, an assertion is triggered with a proper error message.

Besides the tests, it was also used to simplify DRR patterns in the
arith dialect.