[5.0] IRGen: Fix extra inhabitants of multi-payload enums with more spare bits than tag bits. #21068
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AnnaZaks
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…its than tag bits. Because layout minimizes the number of tag bits used, and favors high spare bits, the spare bit representations end up overlapping the extra inhabitant representations, since we just counted down from -1. If there are fewer tag bits than total spare bits, rotate the extra inhabitant values so they correctly line up with the tag representations in this situation. rdar://problem/46468090
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Because layout minimizes the number of tag bits used, and favors high spare bits, the
spare bit representations end up overlapping the extra inhabitant representations, since we
just counted down from -1. If there are fewer tag bits than total spare bits, rotate the
extra inhabitant values so they correctly line up with the tag representations in this
situation. rdar://problem/46468090
This is ABI-impacting (though it shouldn't impact any standard library or overlay types, otherwise we would've noticed this) and necessary for convergence (since it blocks Foundation work).