[llvm] [DWARFLinker][DWARFLinkerParallel][NFC] Refactor DWARFLinker&DWARFLinkerParallel to have a common library. Part 1. (PR #75925)

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avl-llvm wrote:

> > That is perfectly fine for me. Would you like to change command line option also?
> > dsymutil --linker=[classic,parallel]
> > Or it would be only directories/namespaces change only?
> 
> I don't feel strongly about the command line options. We're not using them in production yet so the only churn would be the tests. I'll let you decide whatever you think is best.

I think it would be good to rename options also. Now is the good moment for this(as options are not used widely). Though, I am going to do this in a separate patch. 

https://github.com/llvm/llvm-project/pull/75925