Add order-dependencies to object libraries

[beanz]

Summary

If you are generating an object library that depends on table-gen generate sources, you need the object library to depend on the tablgen target. Currently llvm_add_library doesn't add dependencies for object libraries at all, which is clearly problematic.

[smeenai]

LGTM

llvm/cmake/modules/AddLLVM.cmake

if(CMAKE_GENERATOR STREQUAL "Xcode")
       set_property(SOURCE ${DUMMY_FILE} APPEND_STRING PROPERTY COMPILE_FLAGS "-Wno-empty-translation-unit")
     endif()
     set(ALL_FILES "$<TARGET_OBJECTS:${obj_name}>" ${DUMMY_FILE})
 
     # Do add_dependencies(obj) later due to CMake issue 14747.
     list(APPEND objlibs ${obj_name})
 
     set_target_properties(${obj_name} PROPERTIES FOLDER "Object Libraries")
+    if(ARG_DEPENDS)
+      add_dependencies(${obj_name} ${ARG_DEPENDS})
+    endif()
   endif()
 
   if(ARG_SHARED AND ARG_STATIC)
     # static
     set(name_static "${name}_static")
     if(ARG_OUTPUT_NAME)
       set(output_name OUTPUT_NAME "${ARG_OUTPUT_NAME}")
     endif()