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/compilers/llvm/8/clang/xpc.patch

@@ -0,0 +1,41 @@
+From 61c9b97d7b81cc2c013b423bf1763a92b14fcae3 Mon Sep 17 00:00:00 2001
+From: Jan Korous <jkorous@apple.com>
+Date: Tue, 26 Mar 2019 03:48:25 +0000
+Subject: [PATCH] [clangd][xpc][cmake] Respect explicit value of
+ CLANGD_BUILD_XPC
+
+We shouldn't prevent user from disabling XPC framework build on Darwin.
+However, by keeping it on by default our CI systems also test
+it by default on macOS.
+
+Based on user request:
+http://lists.llvm.org/pipermail/cfe-dev/2019-March/061778.html
+
+Differential Revision: https://reviews.llvm.org/D59808
+
+git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@356974 91177308-0d34-0410-b5e6-96231b3b80d8
+---
+ CMakeLists.txt | 13 ++++++++++---
+ 1 file changed, 10 insertions(+), 3 deletions(-)
+
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index 66ebeaeeaa..514b17fb3c 100644
+--- a/tools/extra/CMakeLists.txt
++++ b/tools/extra/CMakeLists.txt
+@@ -1,6 +1,13 @@
+-option(CLANGD_BUILD_XPC "Build XPC Support For Clangd." OFF)
+-if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+-  set(CLANGD_BUILD_XPC ON CACHE BOOL "" FORCE)
++if (NOT DEFINED CLANGD_BUILD_XPC)
++  if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
++    set(CLANGD_BUILD_XPC_DEFAULT ON)
++  else ()
++    set(CLANGD_BUILD_XPC_DEFAULT OFF)
++  endif ()
++
++  set(CLANGD_BUILD_XPC ${CLANGD_BUILD_XPC_DEFAULT} CACHE BOOL "Build XPC Support For Clangd." FORCE)
++
++  unset(CLANGD_BUILD_XPC_DEFAULT)
+ endif ()
+ 
+ add_subdirectory(clang-apply-replacements)