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Fix compiler crash caused by implicit RHS quantification in type synonyms (#24470)

Andrei Borzenkov requested to merge wip/sand-witch/fix-tysyn into master

Replaced compiler crash with a proper error message:

type SynBad :: forall k. k -> Type
type SynBad = Proxy :: j -> Type
T24470a.hs:7:24: error: [GHC-84925]
    • Type variable ‘j’ in the right-hand side of type synonym ‘SynBad’ is out of arity
    • In the type synonym declaration for ‘SynBad’

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