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/development/tools/castxml/default.nix

@@ -1,7 +1,6 @@
 { lib
 , stdenv
 , fetchFromGitHub
-, clang-unwrapped
 , cmake
 , libclang
 , libffi
@@ -32,13 +31,12 @@ stdenv.mkDerivation rec {
   ];
 
   cmakeFlags = [
-    "-DCLANG_RESOURCE_DIR=${clang-unwrapped}/lib/clang/${lib.getVersion clang-unwrapped}/"
+    "-DCLANG_RESOURCE_DIR=${libclang.dev}/lib/clang/${lib.getVersion libclang}/"
     "-DSPHINX_HTML=${if withHTML then "ON" else "OFF"}"
     "-DSPHINX_MAN=${if withManual then "ON" else "OFF"}"
   ];
 
   buildInputs = [
-    clang-unwrapped
     libffi
     libxml2
     zlib