[dsymutil] Support cloning DIE reference attributes.

Reference attributes are mainly handled by just creating DIEEntry
attributes for them. There is a special case for DW_FORM_ref_addr
attributes though, because the DIEEntry code needs a DwarfDebug
code to emit them (and we don't have one as we do no CodeGen).
In that case, just use DIEInteger attributes with the right form.

llvm-svn: 231531