Note [Type vs Constraint for error ids]

We need both
  iMPOSSIBLE_ERROR_ID            :: forall (r::RuntimeRep) (a::TYPE r).       Addr# -> a
  iMPOSSIBLE_CONSTRAINT_ERROR_ID :: forall (r::RuntimeRep) (a::CONSTRAINT r). Addr# -> a

because we don't have polymorphism over TYPE vs CONSTRAINT.  You
might wonder if iMPOSSIBLE_CONSTRAINT_ERROR_ID is ever needed in
practice, but it is: see #22634.  So:

* In Control.Exception.Base we have
      impossibleError           :: forall (a::Type). Addr# -> a
      impossibleConstraintError :: forall (a::Type). Addr# -> a
  This generates the code for `impossibleError`, but because they are wired in
  the interface file definitions are never looked at (indeed, they don't
  even get serialised).

* In this module GHC.Core.Make we define /wired-in/ Ids for
      iMPOSSIBLE_ERROR_ID
      iMPOSSIBLE_CONSTRAINT_ERROR_ID
   with the desired above types (i.e. runtime-rep polymorphic, and returning a
   constraint for the latter.

Much the same plan works for aBSENT_ERROR_ID and aBSENT_CONSTRAINT_ERROR_ID


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*                                                                      *
                     aBSENT_ERROR_ID
*                                                                      *
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