[llvm-dwp] Emit a rather fictional debug_cu_index

This is very rudimentary support for debug_cu_index, but it is enough to
allow llvm-dwarfdump to find the offsets for  contributions and
correctly dump debug_info.

It will need to actually find the real signature of the unit and build
the real hash table with the right number of buckets, as per the DWP
specification.

It will also need to be expanded to cover the tu_index as well.

llvm-svn: 254489