Fix #10459: SumType should provide convenient access to the type index

[pbackus]

Previously submitted here: #7886
And reverted here: #7922

I'm resubmitting this for the following reasons:

• Users are still asking for this.
• The addition of has and get in Procedural API for SumType #10650 makes typeIndex more useful.
• My response to the @atilaneves's objection in the reversion PR was never acknowledged.
• @LightBender has indicated that he'd be open to merging this if it were resubmitted.

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Testing this PR locally

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[schveiguy]

@atilaneves have your views changed on this?