Get evaluated-ness right in the back end
See Trac #14626, comment:4. We want to maintain evaluted-ness info on Ids into the code generateor for two reasons (see Note [Preserve evaluated-ness in CorePrep] in CorePrep) - DataToTag magic - Potentially using it in the codegen (this is Gabor's current work) But it was all being done very inconsistently, and actually outright wrong -- the DataToTag magic hasn't been working for years. This patch tidies it all up, with Notes to match.
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