[nfc] [lldb] Assertions for D106270 - [DWARF5] Fix offset check when using .debug_names

Skeleton vs. DWO units mismatch has been fixed in D106270. As they both
have type DWARFUnit it is a bit difficult to debug. So it is better to
make it safe against future changes.

Reviewed By: kimanh, clayborg

Differential Revision: https://reviews.llvm.org/D107659