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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> ```
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> ```
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> The following page has more details if needed: [superuser.com/questions/336219/how-do-i-split-a-zip-file-into-multiple-segments](https://superuser.com/questions/336219/how-do-i-split-a-zip-file-into-multiple-segments)
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> [linker-reproducer.zip](https://github.com/llvm/llvm-project/files/13030971/linker-reproducer.zip)
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> [linker-reproducer.z01.zip](https://github.com/llvm/llvm-project/files/13030980/linker-reproducer.z01.zip)

I have confirmed that this patch has actually fixed this regression caused by a previous commit.

Adding a test to prevent regression.

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