[CMake] Set ASM compiler for external projects

[phosek]

Summary

This is necessary on Windows, otherwise CMake fails.

[smeenai]

I don't think it's conventional on Windows to use cl for assembly (you'd use ml or ml64 instead), but apparently CMake has a separate ASM_MASM mode for that, and clang-cl works fine for assembly, so LGTM.

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

if(arg MATCHES "^-DCMAKE_SYSTEM_NAME=")
       string(REGEX REPLACE "^-DCMAKE_SYSTEM_NAME=(.*)$" "\\1" _cmake_system_name "${arg}")
     endif()
   endforeach()
 
   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}
-                          -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang-cl${CMAKE_EXECUTABLE_SUFFIX})
+                          -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang-cl${CMAKE_EXECUTABLE_SUFFIX}
+                          -DCMAKE_ASM_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}
-                          -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang++${CMAKE_EXECUTABLE_SUFFIX})
+                          -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang++${CMAKE_EXECUTABLE_SUFFIX}
+                          -DCMAKE_ASM_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang${CMAKE_EXECUTABLE_SUFFIX})
       endif()
     endif()
     if(lld IN_LIST TOOLCHAIN_TOOLS)
       if(_cmake_system_name STREQUAL Windows)
         list(APPEND compiler_args -DCMAKE_LINKER=${LLVM_RUNTIME_OUTPUT_INTDIR}/lld-link${CMAKE_EXECUTABLE_SUFFIX})
       else()
         list(APPEND compiler_args -DCMAKE_LINKER=${LLVM_RUNTIME_OUTPUT_INTDIR}/ld.lld${CMAKE_EXECUTABLE_SUFFIX})
       endif()