[DebugNames] Implement Entry::GetParentEntry query (#78760)

This commit introduces a helper function to DWARFAcceleratorTable::Entry
which follows DW_IDX_Parent attributes to returns the corresponding
parent Entry in the table.

It is tested by enhancing dwarfdump so that it now prints:

1. When data is corrupt.
2. When parent information is present, but the parent is not indexed.
3. The parent entry offset, when the parent is present and indexed. This
is printed in terms a real entry offset (the same that gets printed at
the start of each entry: "Entry @ 0x..."), instead of the encoded number
in the table (which is an offset from the start off the Entry list).
This makes it easy to visually inspect the dwarfdump and check what the
parent is.