[lldb] [nfc] Simplify user_id_t -> size_t

As discussed in https://reviews.llvm.org/D73206#1871895> simplifying
usage of `user_id_t`.

There is even written:
  // The compile unit ID is the index of the DWARF unit.
  DWARFUnit *dwarf_cu = info->GetUnitAtIndex(comp_unit->GetID());

Differential Revision: https://reviews.llvm.org/D74670