diff --git a/llvm/cmake/modules/LLVMExternalProjectUtils.cmake b/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
--- a/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
+++ b/llvm/cmake/modules/LLVMExternalProjectUtils.cmake
@@ -113,10 +113,12 @@
   if(ARG_USE_TOOLCHAIN AND NOT CMAKE_CROSSCOMPILING)
     if(CLANG_IN_TOOLCHAIN)
       if(_cmake_system_name STREQUAL Windows)
-        set(compiler_args -DCMAKE_C_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang-cl${CMAKE_EXECUTABLE_SUFFIX}
+        set(compiler_args -DCMAKE_ASM_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang-cl${CMAKE_EXECUTABLE_SUFFIX}
+                          -DCMAKE_C_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang-cl${CMAKE_EXECUTABLE_SUFFIX}
                           -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
       else()
-        set(compiler_args -DCMAKE_C_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang${CMAKE_EXECUTABLE_SUFFIX}
+        set(compiler_args -DCMAKE_ASM_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang${CMAKE_EXECUTABLE_SUFFIX}
+                          -DCMAKE_C_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang${CMAKE_EXECUTABLE_SUFFIX}
                           -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang++${CMAKE_EXECUTABLE_SUFFIX})
       endif()
     endif()