[BOLT][DWARF] Add support for DW_IDX_parent (#85285)

This adds support for DW_IDX_parent. If DIE has a parent then
DW_IDX_parent in Entry will point to Entry for that parent DIE.
Otherwise it will have DW_FORM_flag_present in abbrev. Which takes zero
space in Entry.

This came from

https://discourse.llvm.org/t/rfc-improve-dwarf-5-debug-names-type-lookup-parsing-speed/74151