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

[Fangrui Song via llvm-commits]

MaskRay wrote:

For Alexander and others, a previous version of this patch was reviewed internally but it needs many changes to better fit the existing code style. I've suggested that I apply changes on top of this commit stack. 

I am working on a WIP https://github.com/MaskRay/llvm-project/tree/lld-debug-names

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