[lld] [LLD] Tombstone LocalTU entry in .debug_names (PR #70701)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Sorry for not chiming in earlier. I think the current description is
probably inadequate, so a future reader would have a hard time to
analyze this use case.

With `git log --grep debug_names`, I see that #73872 has recently landed
(Dec 4) and now the curious (like I) can redo the experiment.
I am still expecting a concrete example in the patch description (first comment of this PR). https://github.com/llvm/llvm-project/pull/70701#issuecomment-1793910821

I think the description can be rephased like the following:

```
#73872 added support for ...

% cat a.cc
...
% cat b.cc
...
% clang++ -g -gpubnames -fdebug-types-section -fuse-ld=lld a.cc b.cc -o old
% llvm-dwarfdump -v old
....
.... // add a comment that an entry should use the tombstone value
instead.

https://dwarfstd.org/issues/231013.1.html proposes that ...
This patch implements the idea. The new llvm-dwarfdump output will look like:

% llvm-dwarfdump -v new
```


https://github.com/llvm/llvm-project/pull/70701