Fix Trac #3423: missed instantiation for newtype-derived instances
Somehow I'd forgotten to instantiate the coercion that is stored in a 'NewtypeDerived' constructor in an InstInfo. The necessary code is in TcInstDcls.tc_inst_decl2. The result was ghc: panic! (the 'impossible' happened) (GHC version 6.10.3 for x86_64-unknown-linux): No match in record selector Var.tcTyVarDetails because we were looking at an (uninstantiated) TyVar instead of an (instantiated) TcTyVar.
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