gitlab-ci: Disable deb9-llvm job, introduce nightly LLVM job
gitlab-ci: Disable deb9-llvm job, introduce nightly LLVM job

This should help alleviate queue times as the LLVM job is one of the
longest that we have.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc