[commit: ghc] wip/generalized-arrow's head updated: Special case Constraint (9c9fa7f)

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Repository : ssh://git@git.haskell.org/ghc

Branch 'wip/generalized-arrow' now includes:

     84dd9d0 An extra traceTc in tcExpr for ExprWithSig
     356e5e0 Do not eta-reduce across Ticks in CorePrep
     12372ba CorePrep: refactoring to reduce duplication
     067335a A raft of comments about TyBinders
     b416630f Test Trac #11728
     da4bc0c Document implicit quantification better
     454585c More clarification in docs for implicit quantification
     6d0a118 users-guide: Drop TODO in release notes
     1b4750c TysPrim: Generalize kind of (->)
     1d46fbd Fixes
     38de61e Hi
     9c9fa7f Special case Constraint