Since DWARFv5 places TUs in debug_info, some of DWARFContext's APIs have
become a bit erroneous, including TUs in the CU list by accident.
Implementation: Use a filtering iterator when walking the debug_info units looking for CUs.
Drawback: The range type is now different from other unit queries, and the iteration may be slightly slower due to the filtering.
Trying to pick between this solution and D87937