llvm-3.9: move shared libLLVM to a separate output

Fixes #18840: too large closure of mesa_drivers.

Tested atop 16.09:
 - clang compiles a hello-world app;
 - mesa seems to link OK;
 - ispc builds.

Size comparison:
 - 80 MB of full llvm-3.7 on 16.03;
 - 200 MB of full llvm-3.9 on 16.09 before this patch;
 - 50 MB of libLLVM after this commit.

pkgs/development/compilers/llvm/3.9/llvm-outputs.patch

@@ -0,0 +1,26 @@
+diff --git a/tools/llvm-config/llvm-config.cpp b/tools/llvm-config/llvm-config.cpp
+index 94d426b..37f7794 100644
+--- a/tools/llvm-config/llvm-config.cpp
++++ b/tools/llvm-config/llvm-config.cpp
+@@ -333,6 +333,21 @@ int main(int argc, char **argv) {
+     ActiveIncludeOption = "-I" + ActiveIncludeDir;
+   }
+ 
++  /// Nix-specific multiple-output handling: override ActiveLibDir if --link-shared
++  if (!IsInDevelopmentTree) {
++    bool WantShared = true;
++    for (int i = 1; i < argc; ++i) {
++      StringRef Arg = argv[i];
++      if (Arg == "--link-shared")
++        WantShared = true;
++      else if (Arg == "--link-static")
++        WantShared = false; // the last one wins
++    }
++
++    if (WantShared)
++      ActiveLibDir = std::string("@lib@") + "/lib" + LLVM_LIBDIR_SUFFIX;
++  }
++
+   /// We only use `shared library` mode in cases where the static library form
+   /// of the components provided are not available; note however that this is
+   /// skipped if we're run from within the build dir. However, once installed,

pkgs/development/compilers/llvm/3.9/llvm.nix

@@ -30,16 +30,24 @@ in stdenv.mkDerivation rec {
     mv compiler-rt-* $sourceRoot/projects/compiler-rt
   '';
 
+  outputs = [ "out" ] ++ stdenv.lib.optional enableSharedLibraries "lib";
+
   buildInputs = [ perl groff cmake libxml2 python libffi ]
     ++ stdenv.lib.optional stdenv.isDarwin libcxxabi;
 
   propagatedBuildInputs = [ ncurses zlib ];
 
+  postPatch = ""
   # hacky fix: New LLVM releases require a newer OS X SDK than
   # 10.9. This is a temporary measure until nixpkgs darwin support is
   # updated.
-  patchPhase = stdenv.lib.optionalString stdenv.isDarwin ''
+  + stdenv.lib.optionalString stdenv.isDarwin ''
         sed -i 's/os_trace(\(.*\)");$/printf(\1\\n");/g' ./projects/compiler-rt/lib/sanitizer_common/sanitizer_mac.cc
+  ''
+  # Patch llvm-config to return correct library path based on --link-{shared,static}.
+  + stdenv.lib.optionalString (enableSharedLibraries) ''
+    substitute '${./llvm-outputs.patch}' ./llvm-outputs.patch --subst-var lib
+    patch -p1 < ./llvm-outputs.patch
   '';
 
   # hacky fix: created binaries need to be run before installation
@@ -59,7 +67,7 @@ in stdenv.mkDerivation rec {
     "-DLLVM_LINK_LLVM_DYLIB=ON"
   ] ++ stdenv.lib.optional (!isDarwin)
     "-DLLVM_BINUTILS_INCDIR=${binutils.dev}/include"
-    ++ stdenv.lib.optionals ( isDarwin) [
+    ++ stdenv.lib.optionals (isDarwin) [
     "-DLLVM_ENABLE_LIBCXX=ON"
     "-DCAN_TARGET_i386=false"
   ];
@@ -70,7 +78,14 @@ in stdenv.mkDerivation rec {
     paxmark m bin/{lli,llvm-rtdyld}
   '';
 
-  postInstall = stdenv.lib.optionalString (stdenv.isDarwin && enableSharedLibraries) ''
+  postInstall = ""
+  + stdenv.lib.optionalString (enableSharedLibraries) ''
+    moveToOutput "lib/libLLVM-*" "$lib"
+    moveToOutput "lib/libLLVM.so" "$lib"
+    substituteInPlace "$out/lib/cmake/llvm/LLVMExports-release.cmake" \
+      --replace "\''${_IMPORT_PREFIX}/lib/libLLVM-" "$lib/lib/libLLVM-"
+  ''
+  + stdenv.lib.optionalString (stdenv.isDarwin && enableSharedLibraries) ''
     install_name_tool -id $out/lib/libLLVM.dylib $out/lib/libLLVM.dylib
     ln -s $out/lib/libLLVM.dylib $out/lib/libLLVM-${version}.dylib
   '';