Reason: avoid having to link the entire ghc package in modules
that use compile-time annotations:

     import GHC.Exts( SpecConstrAnnotation )
     {-# ANN type T ForceSpecConstr #-}

It's a kind of bug that the package exporting SpecConstrAnnotation
is linked even though it is only needed at compile time, but putting
the data type declaration in GHC.Exts is a simple way to sidestep
the problem

See See Note [SpecConstrAnnotation] in SpecConstr