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This removes the `ConDeclGADTPrefixPs` per the discussion in #18517. Most of this patch simply removes code, although the code in the `rnConDecl` case for `ConDeclGADTPrefixPs` had to be moved around a bit: * The nested `forall`s check now lives in the `rnConDecl` case for `ConDeclGADT`. * The `LinearTypes`-specific code that used to live in the `rnConDecl` case for `ConDeclGADTPrefixPs` now lives in `GHC.Parser.PostProcess.mkGadtDecl`, which is now monadic so that it can check if `-XLinearTypes` is enabled. Fixes #18157.
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