idris and llvm-general

idris 0.9.9 is in master now
And the llvm-general upgrade broke the testphase of these packages.
Also, idris is very specific about the llvm-general version it needs. Jailbreak didn't work (confusion over the split into llvm-general-pure), so I had to bring back the old version and force that to idris.
I don't think any changes in cabal2nix are needed for that though.