Fix #12412 Use enum type for discriminator

As described in #12412, it's reasonably common to define the valid options for a discriminator property as an enum. However, the current versions of the generator assume the discriminator property is always string. This change uses the specified type when it is explicitly defined as a separate type in the schema. I attempted to also handle the scenario where the enum property was specified inline, but it was too complicated for me.

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