llvmPackages*: refactor to use callPackage, etc.

Also use recurseIntoAttrs only on the default version (instead of only on 3.4).

The "self" variants (stil) don't build and they're inconsistent
versions. /cc @shlevy (fea2266290).

pkgs/development/compilers/llvm/3.6/default.nix

@@ -1,4 +1,4 @@
-{ pkgs, newScope, stdenv, isl, fetchurl, overrideCC, wrapCC }:
+{ newScope, stdenv, isl, fetchurl, overrideCC, wrapCC }:
 let
   callPackage = newScope (self // { inherit stdenv isl version fetch; });