Fix dwarf2_string_attr for -gsplit-dwarf

This cherrypicks the upstream commit:
commit 16eb6b2db49e6cf2fdca56efd37689fcc170cd37
Author: Leszek Swirski <leszeks@google.com>
Date:   Mon Aug 7 16:40:38 2017 +0200

Fix dwarf2_string_attr for -gsplit-dwarf

The dwarf2_string_attr did not allow DW_FORM_GNU_str_index as a form for
string types. This manifested as null strings in the namespace_name
lookup (replaced with "(anonymous namespace)") when debugging
Fission-compiled code.

Change-Id: I81ead0d0cad0dce5d40b6d4efa08af60606ef560