[DebugInfo] Include DW_TAG_skeleton_unit when looking for parent UnitDie

`DIE::getUnitDie` looks up parent DIE until compile unit or type unit is found. However for skeleton CU with debug fission, we would have DW_TAG_skeleton_unit instead of DW_TAG_compile_unit as top level DIE.

This change fixes the look up so we can get DW_TAG_skeleton_unit as UnitDie for skeleton CU.

Differential Revision: https://reviews.llvm.org/D120610