FromCabal: remove LLVM evil

There is no reason to tie use of LLVM libraries to the LLVM version
used by GHC’s LLVM backend, especially as the LLVM backend is
usually not being used.

Also, `libclang` is the correct dependency for Clang used as a
library, rather than the wrapped `clang` tool. (In practice, though,
this doesn’t really help: the packages this changes in Nixpkgs
are already broken because they depend on ancient LLVMs, want the
`llvm-config(1)` tool that is in the `llvm` package, and in one of
the two cases the `Setup.hs` doesn’t even build with modern Cabal.)

Author: emilazy

cabal2nix/src/Distribution/Nixpkgs/Haskell/FromCabal.hs

@@ -172,7 +172,7 @@ fromPackageDescription haskellResolver nixpkgsResolver missingDeps flags Package
 
     resolveInNixpkgs :: Identifier -> Binding
     resolveInNixpkgs i
-      | i `elem` ["clang","lldb","llvm"] = binding # (i, path # ["self","llvmPackages",i])     -- TODO: evil!
+      | i == "clang"                     = binding # (i, path # ["pkgs","libclang"])
       | i == "gtk2"                      = binding # (i, path # ["pkgs","gtk2"])               -- TODO: these cases should not be necessary
       | i == "gtk3"                      = binding # (i, path # ["pkgs","gtk3"])
       | i == "gtksourceview3"            = binding # (i, path # ["pkgs","gtksourceview3"])