[LLVM][DWARF] Fix for memory leak

[ayermolo]

This is followup to #8120. Missed a
destuctor.

[felipepiovezan]

Can we just remove the allocator instead and use plain old unique pointers?

[felipepiovezan]

(I'm not familiar with how the abbrev stuff is implemented, so can't speak as to how much time this allocator is saving)

[vitalybuka]

I am going to accept and land to fix the bot.
Please followup with @felipepiovezan .

BTW. std::destroy should work here

[ayermolo]

We can.... I don't really have a strong opinion on this.
It will look something like

std::unique_ptr<DebugNamesAbbrev> NewAbbrev =
            std::make_unique<DebugNamesAbbrev>(std::move(Abbrev));
        NewAbbrev->setNumber(AbbreviationsVector.size() + 1);
        AbbreviationsVector.push_back(std::move(NewAbbrev));
        AbbreviationsSet.InsertNode(AbbreviationsVector.back().get(), InsertPos);
        Value->setAbbrevNumber(AbbreviationsVector.back()->getNumber());

Couple reasons for leaving it like this

1. Bumpptr allocator seems to be a more standard way of dong things, although it can lead to situations like this.
2. This is also how .debug_abbrev handles it. Since we are going for convergence at some point.

[dwblaikie]

Happy with this for now, similar to the debug_info abbrevs - be good to converge the code futher at some point, and see if there's way to do this work better, perhaps.