[CMake] Avoid REVERSE on empty list

[svenvh]

Summary

If required_libs happens to remain empty, CMake would bail out with:

list sub-command REVERSE requires list to be present.

Fix by ensuring we do not attempt to reverse an empty list.

[joey]

LGTM

cmake/modules/LLVM-Config.cmake

function(llvm_expand_dependencies out_libs)
   set(expanded_components ${ARGN})
 
   set(required_libs)
   set(visited_libs)
   foreach( lib ${expanded_components} )
     expand_topologically(${lib} "${required_libs}" "${visited_libs}")
   endforeach()
 
+  if(required_libs)
     list(REVERSE required_libs)
+  endif()
   set(${out_libs} ${required_libs} PARENT_SCOPE)
 endfunction()
 
 function(explicit_map_components_to_libraries out_libs)
   llvm_map_components_to_libnames(link_libs ${ARGN})
   llvm_expand_dependencies(expanded_components ${link_libs})
   # Return just the libraries included in this build:
   set(result)
   foreach(c ${expanded_components})
     if( TARGET ${c} )
       set(result ${result} ${c})
     endif()
   endforeach(c)
   set(${out_libs} ${result} PARENT_SCOPE)
 endfunction(explicit_map_components_to_libraries)