[llvm-dwp] Add support for DWARFv5 type units ...

[llvm-dwp] Add support for DWARFv5 type units                                                                                                                                                                ...

This patch adds support for DWARFv5 type units: parsing from
the .debug_info section, and writing index to the type unit index.
Previously, the type units were part of the .debug_types section
which is no longer used in DWARFv5.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D101818