Make boxed 1-tuples have known keys
Unlike other tuples, which use special syntax and are "known" by way of a special `isBuiltInOcc_maybe` code path, boxed 1-tuples do not use special syntax. Therefore, in order to make sure that the internals of GHC are aware of the `data Unit a = Unit a` definition in `GHC.Tuple`, we give `Unit` known keys. For the full details, see `Note [One-tuples] (Wrinkle: Make boxed one-tuple names have known keys)` in `GHC.Builtin.Types`. Fixes #18097.
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