[cmake] Allow config.guess to be run with MSYS on Windows

[pzheng]

Summary

With r363420, config.guess can no longer be run with MSYS on Windows and this
patch should be able to fix this particular case.

llvm/cmake/modules/GetHostTriple.cmake

if( MSVC )
     endif()
   elseif( MINGW AND NOT MSYS )
     if( CMAKE_SIZEOF_VOID_P EQUAL 8 )
       set( value "x86_64-w64-windows-gnu" )
     else()
       set( value "i686-pc-windows-gnu" )
     endif()
   else( MSVC )
-    if(CMAKE_HOST_SYSTEM_NAME STREQUAL Windows)
+    if(CMAKE_HOST_SYSTEM_NAME STREQUAL Windows AND NOT MSYS)
       message(WARNING "unable to determine host target triple")
     else()
       set(config_guess ${LLVM_MAIN_SRC_DIR}/cmake/config.guess)
       execute_process(COMMAND sh ${config_guess}
         RESULT_VARIABLE TT_RV
         OUTPUT_VARIABLE TT_OUT
         OUTPUT_STRIP_TRAILING_WHITESPACE)
       if( NOT TT_RV EQUAL 0 )
         message(FATAL_ERROR "Failed to execute ${config_guess}")
       endif( NOT TT_RV EQUAL 0 )
       # Defer to dynamic detection of the host AIX version.
       string(REGEX REPLACE "-aix[0-9][^-]*" "-aix" value ${TT_OUT})
     endif()
   endif( MSVC )
   set( ${var} ${value} PARENT_SCOPE )
 endfunction( get_host_triple var )