[llvm] [DWARFLinker][DWARFLinkerParallel] Refactor DWARFLinker&DWARFLinkerParallel to have a common library. (PR #74725)

[via llvm-commits]

avl-llvm wrote:

> High level questions:
> 
>     1. Is this change NFC, or is there some important change reviewers need to be mindful?

This is NFC.

> 
>     2. I see some methods are now virtual when they weren't before and some (String)switch statements have increased in the number of cases they handle. Have you had a chance to double check neither of those is affecting the performance of the linker?

I did not check performance as changed things do not look like they can noticeably affect performance. But I will check and show results.

https://github.com/llvm/llvm-project/pull/74725