llvmPackages: Multuple outputs for everythting

Also begin to start work on cross compilation, though that will have to
be finished later.

The patches are based on the first version of
https://reviews.llvm.org/D99484. It's very annoying to do the
back-porting but the review has uncovered nothing super major so I'm
fine sticking with what I've got.

Beyond making the outputs work, I also strove to re-sync the packages,
as they have been drifting pointlessly apart for some time.

----

Other misc notes, highly incomplete

- lvm-config-native and llvm-config are put in `dev` because they are
  tools just for build time.

- Clang no longer has an lld dep. That was introduced in
  db29857eb3, but if clang needs help
  finding lld when it is used we should just pass it flags / put in the
  resource dir. Providing it at build time increases critical path
  length for no good reason.

----

A note on `nativeCC`:

`stdenv` takes tools from the previous stage, so:

1. `pkgsBuildBuild`: `(?1, x, x)`
2. `pkgsBuildBuild.stdenv.cc`: `(?0, ?1, x)`

while:

1. `pkgsBuildBuild`: `(?1, x, x)`
2. `pkgsBuildBuild.targetPackages`: `(x, x, ?2)`
3. `pkgsBuildBuild.targetPackages.stdenv.cc`: `(?1, x, x)`

pkgs/applications/editors/kdevelop5/kdevelop.nix

@@ -23,7 +23,7 @@ mkDerivation rec {
 
   buildInputs = [
     kdevelop-pg-qt
-    llvmPackages.llvm llvmPackages.clang-unwrapped
+    llvmPackages.llvm llvmPackages.libclang
   ];
 
   propagatedBuildInputs = [
@@ -37,7 +37,7 @@ mkDerivation rec {
   # https://cgit.kde.org/kdevelop.git/commit/?id=716372ae2e8dff9c51e94d33443536786e4bd85b
   # required as nixos seems to be unable to find CLANG_BUILTIN_DIR
   cmakeFlags = [
-    "-DCLANG_BUILTIN_DIR=${llvmPackages.clang-unwrapped}/lib/clang/${lib.getVersion llvmPackages.clang}/include"
+    "-DCLANG_BUILTIN_DIR=${llvmPackages.libclang.lib}/lib/clang/${lib.getVersion llvmPackages.clang}/include"
   ];
 
   dontWrapQtApps = true;