DebugInfo: Filter DWARFv5 TUs out of the debug_info unit list when CUs requested

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.
Correct that by providing compile_units (& dwo_compile_units) that
filter out the type units from the debug_info units.

Differential Revision: https://reviews.llvm.org/D87935