clang8: apply patch to disable xpc

pkgs/development/compilers/llvm/8/clang/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)

pkgs/development/compilers/llvm/8/clang/default.nix

@@ -24,6 +24,7 @@ let
 
     cmakeFlags = [
       "-DCMAKE_CXX_FLAGS=-std=c++11"
+      "-DCLANGD_BUILD_XPC=OFF"
     ] ++ stdenv.lib.optionals enableManpages [
       "-DCLANG_INCLUDE_DOCS=ON"
       "-DLLVM_ENABLE_SPHINX=ON"
@@ -35,7 +36,10 @@ let
       "-DLINK_POLLY_INTO_TOOLS=ON"
     ];
 
-    patches = [ ./purity.patch ];
+    patches = [
+      ./purity.patch
+      ./clang-xpc.patch
+    ];
 
     postPatch = ''
       sed -i -e 's/DriverArgs.hasArg(options::OPT_nostdlibinc)/true/' \