[CMake][compiler-rt] Support clang-cl in CompilerRTMockLLVMCMakeConfig

clang-cl doesn't support -dumpmachine directly, so we need to
preface it with /clang: in order to get this probing function
to work.

This is needed in order to run cmake directly on the runtimes
directory.

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D143557

compiler-rt/cmake/Modules/CompilerRTMockLLVMCMakeConfig.cmake

@@ -46,8 +46,14 @@ function(compiler_rt_mock_llvm_cmake_config_set_target_triple)
     if ("${CMAKE_C_COMPILER_ID}" MATCHES "Clang|GNU")
       # Note: Clang also supports `-print-target-triple` but gcc doesn't
       # support this flag.
+      set(DUMPMACHINE_ARG -dumpmachine)
+      if(MSVC)
+        # This means we are using clang-cl and it requires
+        # /clang: as prefix for dumpmachine argument.
+        set(DUMPMACHINE_ARG /clang:-dumpmachine)
+      endif()
       execute_process(
-        COMMAND "${CMAKE_C_COMPILER}" -dumpmachine
+        COMMAND "${CMAKE_C_COMPILER}" ${DUMPMACHINE_ARG}
         RESULT_VARIABLE HAD_ERROR
         OUTPUT_VARIABLE COMPILER_OUTPUT
         OUTPUT_STRIP_TRAILING_WHITESPACE)