VS: Support Visual Studio Clang Toolkit identification

Teach CMake that the `ClangCl` toolset uses the `ClangClExecutable`
value as the path to the compiler executable.

Modules/CMakeDetermineCompilerId.cmake

@@ -246,7 +246,7 @@ Id flags: ${testflags} ${CMAKE_${lang}_COMPILER_ID_FLAGS_ALWAYS}
     set(id_platform ${CMAKE_VS_PLATFORM_NAME})
     set(id_lang "${lang}")
     set(id_PostBuildEvent_Command "")
-    if(CMAKE_VS_PLATFORM_TOOLSET MATCHES "^[Ll][Ll][Vv][Mm](_v[0-9]+(_xp)?)?$")
+    if(CMAKE_VS_PLATFORM_TOOLSET MATCHES "^([Ll][Ll][Vv][Mm](_v[0-9]+(_xp)?)?|[Cc][Ll][Aa][Nn][Gg][Cc][Ll])$")
       set(id_cl_var "ClangClExecutable")
     elseif(CMAKE_VS_PLATFORM_TOOLSET MATCHES "v[0-9]+_clang_.*")
       set(id_cl clang.exe)