Manual DWARF index: don't skip over -gmodules debug info

This fixes a regression introduced by
https://reviews.llvm.org/D131437. The intention of the patch was to
avoid indexing DWO skeleton units, but it also skipped over full DWARF
compile units linked via a -gmodules DW_AT_dwo_name attribute. This
patch restores the functionality and adds a test for it.

Differential Revision: https://reviews.llvm.org/D142683