DWO ID from the unit DIE/header for split DWARF, if present.
Declared in <llvm/DebugInfo/DWARF/DWARFUnit.h>
<llvm/DebugInfo/DWARF/DWARFUnit.h>
std::optional<uint64_t> getDWOId();
DWO ID for split DWARF, or std::nullopt if absent.