toIfaceIdDetails hits an unexpected case.

I saw output from a stray ppr in my build:

| Run Hsc2Hs Stage1: libraries/unix/System/Posix/Terminal.hsc => _devel/stage1/libraries/unix/build/System/Posix/Terminal.hs
toIfaceIdDetails [RepPolyId]
| Run Ghc CompileHs Stage1: libraries/base/GHC/ArrayArray.hs => _devel/stage1/libraries/base/build/GHC/ArrayArray.o

Which seems to come from:

  -- The remaining cases are all "implicit Ids" which don't
  -- appear in interface files at all
toIfaceIdDetails other = pprTrace "toIfaceIdDetails" (ppr other)
                         IfVanillaId   -- Unexpected; the other

I think we need to update this to account for RepPolyId.

@sheaf who added RepPolyId