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This patch cleans up the complexity around WW's `mk_absent_let` by broadening the scope of `LitRubbish`. Rubbish literals now store the `PrimRep` they represent and are ultimately lowered in Cmm. This in turn allows absent literals of `VecRep` or `VoidRep`. The latter allows absent literals for unlifted coercions, as requested in #18983. I took the liberty to rewrite and clean up `Note [Absent fillers]` and `Note [Rubbish values]` to account for the new implementation and to make them more orthogonal in their description. I didn't add a new regression test, as `T18982` already contains the test in the ticket and its test output changes as expected. Fixes #18983.
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