[CMake] Correct CMake message mode

[xiaobai]

Summary

This wasn't actually printing out a CMake warning, it was prepending
"WARN" to the message.

[smeenai]

I think this is small and obvious enough that you could just commit directly for post-commit review :)

llvm/trunk/cmake/modules/AddLLVM.cmake

if (ARG_ALLOW_EXTERNAL)
     if (NOT "${LLVM_EXTERNAL_LIT}" STREQUAL "")
       if (EXISTS ${LLVM_EXTERNAL_LIT})
         get_filename_component(LIT_FILE_NAME ${LLVM_EXTERNAL_LIT} NAME)
         get_filename_component(LIT_BASE_DIR ${LLVM_EXTERNAL_LIT} DIRECTORY)
         set(${file_name} ${LIT_FILE_NAME} PARENT_SCOPE)
         set(${base_dir} ${LIT_BASE_DIR} PARENT_SCOPE)
         return()
       else()
-        message(WARN "LLVM_EXTERNAL_LIT set to ${LLVM_EXTERNAL_LIT}, but the path does not exist.")
+        message(WARNING "LLVM_EXTERNAL_LIT set to ${LLVM_EXTERNAL_LIT}, but the path does not exist.")
       endif()
     endif()
   endif()
 
   set(lit_file_name "llvm-lit")
   if (CMAKE_HOST_WIN32 AND NOT CYGWIN)
     # llvm-lit needs suffix.py for multiprocess to find a main module.
     set(lit_file_name "${lit_file_name}.py")