[DWARF5] Fix offset check when using .debug_names

When going through the CU entries in the name index,
make sure to compare the name entry's CU
offset against the skeleton CU's offset.

Previously there would be a mismatch, since the
wrong offset was compared, and thus no suitable
entry was found.

Reviewed By: jankratochvil

Differential Revision: https://reviews.llvm.org/D106270