Add missing kind cast to pure unifier.
Add missing kind cast to pure unifier.

(cherry picked from commit aff5bb47b70450bb1e3e4ac3c18ea35d13f9ac7c)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc