[lldb/DWARF] Support the debug_str_offsets section in dwp files

Summary:
In dwp files a constant (from the debug_cu_index section) needs to be
added to each reference into the debug_str_offsets section.

I've tried to implement this to roughly match the llvm flow: I've
changed the DWARFormValue to stop resolving the indirect string
references directly -- instead, it calls into DWARFUnit, which resolves
this for it (similar to how it already resolves indirect range and
location list references). I've also done a small refactor of the string
offset base computation code in DWARFUnit in order to make it easier to
access the debug_cu_index base offset.

Reviewers: JDevlieghere, aprantl, clayborg

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D74723