[cmake] restrict object library dependency generation to PUBLIC libraries

[stephenneuendorffer]

Summary

Previous patch broken flang, which has some yet-to-be resolved cyclic
dependencies. This patch fixes the breakage by restricting the dependencies
which are generated to public libraries, which is probably more sensible anyway.

llvm/cmake/modules/AddLLVM.cmake

if((ARG_SHARED AND ARG_STATIC) OR ARG_OBJECT)
     set_target_properties(${obj_name} PROPERTIES FOLDER "Object Libraries")
     if(ARG_DEPENDS)
       add_dependencies(${obj_name} ${ARG_DEPENDS})
     endif()
     # Treat link libraries like PUBLIC dependencies.  LINK_LIBS might
     # result in generating header files.  Add a dependendency so that
     # the generated header is created before this object library.
     if(ARG_LINK_LIBS)
-      foreach(link_lib ${ARG_LINK_LIBS})
+      cmake_parse_arguments(LINK_LIBS_ARG
+        ""
+        ""
+        "PUBLIC;PRIVATE"
+        ${ARG_LINK_LIBS})
+      foreach(link_lib ${LINK_LIBS_ARG_PUBLIC})
         if(TARGET ${link_lib})
           add_dependencies(${obj_name} ${link_lib})
         endif()
       endforeach()
     endif()
   endif()
 
   if(ARG_SHARED AND ARG_STATIC)