[lld] [lld][ELF] Implement merged .debug_names section. (PR #86508)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Except for maskray's request re using 'rm -rf %t && split-file %s %t && cd %t' in tests (still looking into that), I think I have addressed all the review request/comments. So it's ready for any additional reviewing.
> 
> If I've overlooked something please let me know.
> 
> Re @ayermolo 's suggestion to pull the DebugNames work out of SyntheticSections.cpp into another file: That's a decision I would want to leave to @MaskRay . I don't know how much doing so would really help, however. The DebugNamesSection work in SyntheticSections.cpp is only ~750 lines, and it's all contiguous...

Adding a new file might require to make some internal functions (`static`) external. There can be more complexity if the new file needs to call a template. I think we should avoid a new file for now.
That said, ~750 lines are a lot of complexity. I'll check if I can simplify the code.

https://github.com/llvm/llvm-project/pull/86508