[DWARF] Check for AddrOffsetSectionBase to work with DWO Units.

Context: https://lists.llvm.org/pipermail/llvm-dev/2021-February/148521.html

A fix for llvm-symbolizer, and other tools like BOLT, that allows retrieving address when built with -gsplit-dwarf=single mode.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D96827