[DWARF][NFC] add ParentIdx and SiblingIdx to DWARFDebugInfoEntry for faster navigation.

This patch implements suggestion done while reviewing D102634. It adds two fields:
ParentIdx and SiblingIdx. These fields allow fast navigation to die parent and
die sibling. These fields are set at the moment when dies are loaded.

dsymutil works 2% faster with this patch(run on clang binary).

Differential Revision: https://reviews.llvm.org/D110363