34a8e6eee666 changed SymbolFileDWARF::GetDwoNum to
SymbolFileDWARF::GetFileIndex but changed the meaning from just DWO to
DWO and OSO which changed the meaning of the assert. The assert was
therefore removed from ManualDWARFIndex::GetGlobalVariables and
ManualDWARFIndex::GetGlobalVariables but was still present in
DebugNamesDWARFIndex::GetGlobalVariables. If we want to reintroduce the
assert, we need something with the old semantics for all 3.
048204d610