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/12/lldb/default.nix

@@ -1,4 +1,5 @@
 { lib, stdenv, llvm_meta
+, runCommand
 , fetch
 , cmake
 , zlib
@@ -7,8 +8,8 @@
 , which
 , libedit
 , libxml2
-, llvm
-, clang-unwrapped
+, libllvm
+, libclang
 , python3
 , version
 , libobjc
@@ -27,17 +28,30 @@ stdenv.mkDerivation (rec {
 
   src = fetch pname "1v85qyq3snk81vjmwq5q7xikyyqsfpqy2c4qmr81mps4avsw1g0l";
 
-  patches = [ ./lldb-procfs.patch ];
+  patches = [
+    ./procfs.patch
+    (runCommand "resource-dir.patch" {
+      clangLibDir = "${libclang.lib}/lib";
+    } ''
+      substitute '${./resource-dir.patch}' "$out" --subst-var clangLibDir
+    '')
+    ./gnu-install-dirs.patch
+  ];
 
-  nativeBuildInputs = [ cmake python3 which swig lit ]
-    ++ lib.optionals enableManpages [ python3.pkgs.sphinx python3.pkgs.recommonmark ];
+  outputs = [ "out" "lib" "dev" ];
+
+  nativeBuildInputs = [
+    cmake python3 which swig lit
+  ] ++ lib.optionals enableManpages [
+    python3.pkgs.sphinx python3.pkgs.recommonmark
+  ];
 
   buildInputs = [
     ncurses
     zlib
     libedit
     libxml2
-    llvm
+    libllvm
   ]
   ++ lib.optionals stdenv.isDarwin [
     libobjc
@@ -51,8 +65,9 @@ stdenv.mkDerivation (rec {
   hardeningDisable = [ "format" ];
 
   cmakeFlags = [
+    "-DLLDB_INCLUDE_TESTS=${if doCheck then "YES" else "NO"}"
     "-DLLVM_ENABLE_RTTI=OFF"
-    "-DClang_DIR=${clang-unwrapped}/lib/cmake"
+    "-DClang_DIR=${libclang.dev}/lib/cmake"
     "-DLLVM_EXTERNAL_LIT=${lit}/bin/lit"
   ] ++ lib.optionals stdenv.isDarwin [
     "-DLLDB_USE_SYSTEM_DEBUGSERVER=ON"
@@ -62,8 +77,13 @@ stdenv.mkDerivation (rec {
     "-DLLVM_ENABLE_SPHINX=ON"
     "-DSPHINX_OUTPUT_MAN=ON"
     "-DSPHINX_OUTPUT_HTML=OFF"
+  ] ++ lib.optionals doCheck [
+    "-DLLDB_TEST_C_COMPILER=${stdenv.cc}/bin/${stdenv.cc.targetPrefix}cc"
+    "-DLLDB_TEST_CXX_COMPILER=${stdenv.cc}/bin/${stdenv.cc.targetPrefix}c++"
   ];
 
+  doCheck = false;
+
   postInstall = ''
     # Editor support
     # vscode:
@@ -90,6 +110,7 @@ stdenv.mkDerivation (rec {
   '';
 
   propagatedBuildInputs = [];
+
   # manually install lldb man page
   installPhase = ''
     mkdir -p $out/share/man/man1