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

[thieta]

Summary

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.

[thetruestblue]

This seems reasonable and fine to me.
Though I'd feel better if someone familiar with MSVC/Windows gave approval.

compiler-rt/cmake/Modules/CompilerRTMockLLVMCMakeConfig.cmake

52

NIT: Single line comment suggestion:

# Use /clang:-dumpmachine for clang-cl.

[hans]

I'm not familiar with this cmake code, but it seems reasonable to lgtm if it works :)

[thieta]

In D143557#4113334, @thetruestblue wrote:

This seems reasonable and fine to me.
Though I'd feel better if someone familiar with MSVC/Windows gave approval.

Whoops forgot your comment suggestion. Fixed that after I had landed this and pushed it as a NFC. Thanks!

compiler-rt/cmake/Modules/CompilerRTMockLLVMCMakeConfig.cmake

function(compiler_rt_mock_llvm_cmake_config_set_target_triple)
   endif()
 
   # Try asking the compiler for its default target triple.
   set(HAD_ERROR FALSE)
   if ("${COMPILER_OUTPUT}" STREQUAL "")
     if ("${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}" ${DUMPMACHINE_ARG}
         RESULT_VARIABLE HAD_ERROR
         OUTPUT_VARIABLE COMPILER_OUTPUT
         OUTPUT_STRIP_TRAILING_WHITESPACE)
     else()
       message(FATAL_ERROR
         "Fetching target triple from compiler \"${CMAKE_C_COMPILER_ID}\" "
         "is not implemented.")
     endif()