Use llvm-hs/llvm-hs@llvm-9 in cabal.project.
Use llvm-hs/llvm-hs@llvm-9 in cabal.project.

Previously, a downstream commit at apaszke/llvm-hs was referenced.
That commit has now been merged into llvm-hs/llvm-hs@llvm-9.

google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang,google-research/dex-lang