build: use clang-cl for runtimes when targeting Windows

[compnerd]

Summary

Adjust the CMAKE_C{,XX}_COMPILER passed to the subproject build for the runtimes when targeting Windows.

[beanz]

LGTM.

[plotfi]

Note: I think we should probably do something about the linker here too. so use lld-link instead of ld.lld in the Windows case.

[compnerd]

update for linker as well and the non-primary builtins

[beanz]

LGTM. Makes sense.

[compnerd]

SVN r361072

cmake/modules/LLVMExternalProjectUtils.cmake

     endforeach()
   endforeach()
 
+  foreach(arg ${ARG_CMAKE_ARGS})
+    if(arg MATCHES "^-DCMAKE_SYSTEM_NAME=")
+      string(REGEX REPLACE "^-DCMAKE_SYSTEM_NAME=(.*)$" "\\1" _cmake_system_name "${arg}")
+    endif()
+  endforeach()
+
   if(ARG_USE_TOOLCHAIN AND NOT CMAKE_CROSSCOMPILING)
     if(CLANG_IN_TOOLCHAIN)
-      set(compiler_args -DCMAKE_C_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang
-                        -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang++)
+      if(_cmake_system_name STREQUAL Windows)
+        set(compiler_args -DCMAKE_C_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang-cl
+                          -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang-cl)
+      else()
+        set(compiler_args -DCMAKE_C_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang
+                          -DCMAKE_CXX_COMPILER=${LLVM_RUNTIME_OUTPUT_INTDIR}/clang++)
+      endif()
     endif()
     if(lld IN_LIST TOOLCHAIN_TOOLS)
-      list(APPEND compiler_args -DCMAKE_LINKER=${LLVM_RUNTIME_OUTPUT_INTDIR}/ld.lld)
+      if(_cmake_system_name STREQUAL Windows)
+        list(APPEND compiler_args -DCMAKE_LINKER=${LLVM_RUNTIME_OUTPUT_INTDIR}/lld-link)
+      else()
+        list(APPEND compiler_args -DCMAKE_LINKER=${LLVM_RUNTIME_OUTPUT_INTDIR}/ld.lld)
+      endif()
     endif()
     if(llvm-ar IN_LIST TOOLCHAIN_TOOLS)
       list(APPEND compiler_args -DCMAKE_AR=${LLVM_RUNTIME_OUTPUT_INTDIR}/llvm-ar)