Revert "Revert "Revert "Revert "DebugInfo: Omit debug info for dynamic classes in TUs that do not have the vtable for that class""""

This reverts commit r188687 (reverts r188642 (reverts 188600 (reverts
188576))).

With added test coverage & fix for -gline-tables-only.

Thanks Michael Gottesman for reverting this patch when it demonstrated
problems & providing a reproduction/details to help me track this down.