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/networking/browsers/firefox/common.nix

@@ -248,8 +248,8 @@ buildStdenv.mkDerivation ({
       $(< ${buildStdenv.cc}/nix-support/libc-cflags) \
       $(< ${buildStdenv.cc}/nix-support/cc-cflags) \
       $(< ${buildStdenv.cc}/nix-support/libcxx-cxxflags) \
-      ${lib.optionalString buildStdenv.cc.isClang "-idirafter ${buildStdenv.cc.cc}/lib/clang/${lib.getVersion buildStdenv.cc.cc}/include"} \
-      ${lib.optionalString buildStdenv.cc.isGNU "-isystem ${buildStdenv.cc.cc}/include/c++/${lib.getVersion buildStdenv.cc.cc} -isystem ${buildStdenv.cc.cc}/include/c++/${lib.getVersion buildStdenv.cc.cc}/${buildStdenv.hostPlatform.config}"} \
+      ${lib.optionalString buildStdenv.cc.isClang "-idirafter ${buildStdenv.cc.cc.lib}/lib/clang/${lib.getVersion buildStdenv.cc.cc}/include"} \
+      ${lib.optionalString buildStdenv.cc.isGNU "-isystem ${lib.getDev buildStdenv.cc.cc}/include/c++/${lib.getVersion buildStdenv.cc.cc} -isystem ${buildStdenv.cc.cc}/include/c++/${lib.getVersion buildStdenv.cc.cc}/${buildStdenv.hostPlatform.config}"} \
       $NIX_CFLAGS_COMPILE"
 
     echo "ac_add_options BINDGEN_CFLAGS='$BINDGEN_CFLAGS'" >> $MOZCONFIG
@@ -282,7 +282,7 @@ buildStdenv.mkDerivation ({
     "--disable-updater"
     "--enable-jemalloc"
     "--enable-default-toolkit=${default-toolkit}"
-    "--with-libclang-path=${llvmPackages.libclang}/lib"
+    "--with-libclang-path=${llvmPackages.libclang.lib}/lib"
     "--with-system-nspr"
     "--with-system-nss"
   ]