Add type parameters derived from enum to default param getters of enum cases

[kasiaMarek]

fixes: #22137

[dwijnand]

I almost feel like we should fix the constructor, by giving it type parameters, afterwhich the default getters would have the type parameters. But going straight from contextual enum class type parameters to modifying the default getters also works (though we're skipping the dropContextBounds call, so that might matter eventually).

[kasiaMarek]

though we're skipping the dropContextBounds call, so that might matter eventually

I assumed it's might be fine to skip it since rhs is always DerivedFromParamTree for these type params.

[SethTisue]

Nice — thanks for the quick fix 👍