Apply patch to use proper clang

cmake-fix.patch

@@ -0,0 +1,45 @@
+diff --git a/cmake/LLVM.cmake b/cmake/LLVM.cmake
+index 11472060a6..0e570f5a21 100644
+--- a/cmake/LLVM.cmake
++++ b/cmake/LLVM.cmake
+@@ -335,7 +335,6 @@
+     "${PROG_NAME}${CMAKE_EXECUTABLE_SUFFIX}"
+     HINTS "${LLVM_BINDIR}" "${LLVM_CONFIG_LOCATION}"
+     DOC "${DOCSTRING}"
+-    NO_DEFAULT_PATH
+     NO_CMAKE_PATH
+     NO_CMAKE_ENVIRONMENT_PATH
+   )
+@@ -346,7 +345,12 @@
+   endif()
+ endmacro()
+ 
+-find_program_or_die(CLANG "clang" "clang binary")
++if(DEFINED CMAKE_C_COMPILER) # TODO: this assumes we are using clang
++  set(CLANG "${CMAKE_C_COMPILER}")
++  message(STATUS "Using clang: ${CLANG}")
++else()
++  find_program_or_die(CLANG "clang" "clang binary")
++endif()
+ execute_process(COMMAND "${CLANG}" "--version" OUTPUT_VARIABLE LLVM_CLANG_VERSION RESULT_VARIABLE CLANG_RES)
+ find_program_or_die(CLANGXX "clang++" "clang++ binary")
+ execute_process(COMMAND "${CLANGXX}" "--version" OUTPUT_VARIABLE LLVM_CLANGXX_VERSION RESULT_VARIABLE CLANGXX_RES)
+@@ -925,9 +929,15 @@
+ 
+ #####################################################################
+ 
+-execute_process(COMMAND "${CLANG}" "--print-resource-dir" OUTPUT_VARIABLE RESOURCE_DIR)
+-string(STRIP "${RESOURCE_DIR}" RESOURCE_DIR)
+-set(CLANG_RESOURCE_DIR "${RESOURCE_DIR}" CACHE INTERNAL "Clang resource dir")
++if (NOT DEFINED CLANG_RESOURCE_DIR)
++  execute_process(COMMAND "${CLANG}" "--print-resource-dir" OUTPUT_VARIABLE RESOURCE_DIR)
++  string(STRIP "${RESOURCE_DIR}" RESOURCE_DIR)
++  set(CLANG_RESOURCE_DIR "${RESOURCE_DIR}" CACHE INTERNAL "Clang resource dir")
++endif()
++
++set(CLANG_OPENCL_HEADERS "${CLANG_RESOURCE_DIR}/include/opencl-c.h"
++		"${CLANG_RESOURCE_DIR}/include/opencl-c-base.h")
++message(STATUS "clang OpenCL Headers: ${CLANG_OPENCL_HEADERS}")
+ 
+ ##########################################################
+ 

default.nix

@@ -45,6 +45,8 @@ llvmPackages.stdenv.mkDerivation (finalAttrs: {
   version = "nightly";
   inherit src;
 
+  patches = [ ./cmake-fix.patch ];
+
   cmakeFlags = [
     # TODO: all these are broken when cross compiling. Upstream has refactored
     # all the cmake infra for cross compilation, but it's not in a release yet