Handle TagToEnum in the same big case as the other primops

Before, it was a panic because it was handled above. But there must have been an error in my reasoning (another caller?) because #17442 (closed) reported the panic was hit.

But, rather than figuring out what happened, I can just make it impossible by construction. By adding just a bit more bureaucracy in the return types, I can handle TagToEnum in the same case as all the others, so the big case is is now total, and the panic is removed.

Fixes #17442 (closed)

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