[DebugInfo] Emit skeleton to avoid mismatching inlining flags (#153568)
This actually reverts 418120556398c01550d42500d56e6d328290185b.
The original commit omits unit with all symbols inlined into current
one, which leads to crash when a module using split-dwarf inlined a
function from another module with mismatched split-dwarf-inlining
option. This revert guarantees that DIEs are created in both DWO and the
skeleton sections whenever split-dwarf is active.