DWARF v5 - compiler work update

At EuroLLVM's Debug Info BoF, I made a rash statement about being done
with DWARF v5 "next week." It has been an impressively long week...
and it's not over yet. But given Pavel Labath's awesome news about
accelerator tables, it seemed like a good moment to review the rest of
the DWARF v5 work.

This really falls into three broad categories:
- the bare minimum needed to be conformant with v5;
- making some optional features conformant with v5;
- other things that are pretty useful or beneficial;
- everything else.

Because of unexpected complexities and other factors, the DWARF v5
work has generally been taking longer than expected. I am still
hopeful to get to the "minimally conformant" stage before we branch
LLVM 7.0 on the first of August. Some other useful/beneficial things
have already gone in, and that's also really positive. But there is
quite a bit left to go.

I apologize in advance for not crediting people who did the work,
or (just as important) all the reviewing! All efforts are greatly

Minimally Conformant Features