[lld] [ELF] Merge copyLocalSymbols and demoteLocalSymbolsInDiscardedSections (PR #69425)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> After bisecting it seems like [bbf7b9d](https://github.com/llvm/llvm-project/commit/bbf7b9d805f5773b4fe5bfb69081fe3691a90fb5) is when the issue crops up.
> 
> Sorry for pinging the wrong PR. Our CI infra hasn't adapted to linkify github PR numbers yet, and I must have copied the wrong PR number. At least I pinged the right person, but still not great. I'll file a separate issue and CC you directly.

I saw your internal message as well. Do you mean that

bbf7b9d805f5773b4fe5bfb69081fe3691a90fb5 ("[ELF] Remove unused setSymbolAndType after https://github.com/llvm/llvm-project/pull/69295. NFC") introduced a regression which is fixed by this patch?

This ensures for every `Defined` symbol, `d->section->isLive` is true. The "NFC" part from the commit message bbf7b9d805f5773b4fe5bfb69081fe3691a90fb5 is incorrect. 

Thank you for the zipped reproduce files. I think we miss some coverage for --emit-relocs or -r.

https://github.com/llvm/llvm-project/pull/69425