Fix LLVM target triple computation with MSVC

[zturner]

Summary

CMake tries to determine the host target triple for the build by checking the value of CMAKE_CL_64. The value of this variable is tied to the CMake generator, and not the toolchain in use. In particular, it is only set to 1 when a generator of the form "Visual Studio <n> Win64" is used. This means that it doesn't work for the Ninja generator, and will always give you a host triple of i686-pc-win32 even when compiling with a 64-bit toolchain.

This patch addresses this by running ${CMAKE_CXX_COMPILER} when MSVC is detected, and parsing its output to determine the architecture that it claims it targets, and then choosing the target triple accordingly.

[chandlerc]

Assuming you'le already handled this? Put me back on if this actually still needs review.

cmake/modules/GetHostTriple.cmake

 # Returns the host triple.
 # Invokes config.guess
 
 function( get_host_triple var )
   if( MSVC )
-    if( CMAKE_CL_64 )
+    execute_process(
+      COMMAND ${CMAKE_CXX_COMPILER}
+      OUTPUT_VARIABLE CL_OUTPUT
+      ERROR_VARIABLE CL_ERROR)
+    STRING(REGEX REPLACE "Microsoft \\(R\\).* for (x64|x86|ARM).*"
+                         "\\1" 
+                         CL_TARGET_ARCHITECTURE
+                         ${CL_ERROR})
+    if ( "${CL_TARGET_ARCHITECTURE}" STREQUAL "x64" )
       set( value "x86_64-pc-win32" )
-    else()
+    elseif ( "${CL_TARGET_ARCHITECTURE}" STREQUAL "ARM" )
+      set( value "arm-pc-win32" )
+    else ()
       set( value "i686-pc-win32" )
     endif()
   elseif( MINGW AND NOT MSYS )
     if( CMAKE_SIZEOF_VOID_P EQUAL 8 )
       set( value "x86_64-w64-mingw32" )
     else()
       set( value "i686-pc-mingw32" )
     endif()