[cmake] Ensure that LINK_LIBS are dependencies for object library targets

[stephenneuendorffer]

Summary

In MLIR, it is common for automatically generated headers to be included
in many places. To avoid tracking these dependencies explicitly in
cmake, they are treated as part of a library which 'owns' the generated
header. Users of the generated header link against the owning library.
However, object libraries don't actually 'link', so this dependence gets
lost. This patch adds an explicit dependence for these generated headers
when creating object library targets to ensure that generated headers
are appropriately generated

llvm/cmake/modules/AddLLVM.cmake

if((ARG_SHARED AND ARG_STATIC) OR ARG_OBJECT)
 
     # 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()
+    # 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})
+        if(TARGET ${link_lib})
+          add_dependencies(${obj_name} ${link_lib})
+        endif()
+      endforeach()
+    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()