[DebugInfo] Fix reading DWARFv5 type units in DWP.

In DWARFv5, type units are stored in .debug_info sections, along with
compilation units, and they are distinguished by the unit_type field
in the header, not by the name of the section. It is impossible to
associate the correct index section of a DWP file with the unit before
the unit's header is read. This patch fixes reading DWARFv5 type units
by parsing the header first and then applying the index entry according
to the actual unit type.

Differential Revision: https://reviews.llvm.org/D77552