[DWARFLinker] Emit DW_IDX_parent in the accelerator table (#195403) .debug_names entries produced by the parallel linker were always emitted with std::nullopt for ParentDIEOffset, resulting in a missing DW_IDX_parent. The classic linker emits it via DWARF5AccelTableData::getDefiningParentDieOffset on the output DIE tree. The parallel linker can't use the same approach because the records are saved during cloneDIE, before the output DIE has been linked into its parent, so DIE::getParent() is nullptr at that time time. Fix that by computing the parent offset from the input-side DIE tree instead. We look up InputDieEntry's parent via getParentIdx, skip parents marked DW_AT_declaration, and translate them to the output offset through CompileUnit::getDieOutOffset. Since no real DIE can live at offset 0, we can use that to unambiguously mark input DIEs that were not cloned into this CU's plain DWARF (e.g. routed only into the artificial type unit) and is treated as "no parent". Only compile-unit accelerator entries are covered. Type-unit entries (artificial type unit) still emit no DW_IDX_parent, tracked by a TODO.
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