### Expand type/kind synonyms in TyVars before deriving-related typechecking

Before, it was possible to have a datatypes such as ``` type ConstantT a b = a newtype T (f :: * -> *) (a :: ConstantT * f) = T (f a) deriving Functor data family TFam (f :: * -> *) (a :: *) newtype instance TFam f (ConstantT a f) = TFam (f a) deriving Functor ``` fail to eta-reduce because either (1) a TyVar had a kind synonym that mentioned another TyVar, or (2) an instantiated type was itself a type synonym that mentioned another TyVar. A little bit of tweaking to `expandTypeSynonyms` and applying it before the eta-reduction check in the `deriving` machinery is sufficient to fix this. Fixes #11416. Test Plan: ./validate Reviewers: goldfire, simonpj, austin, bgamari Reviewed By: simonpj Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1772 GHC Trac Issues: #11416

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