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

[Jonas Devlieghere via llvm-commits]

JDevlieghere wrote:

The base class looks good but I would prefer a slight change to the directory layout and namespaces. Particularly the distinction between "Apple" and "LLVM" bother me. I understand how you ended up there, but I think it unnecessary associates the algorithm with particular vendors. How about "Parallel" and "Classic"? 

With that naming scheme, what I'd like this to look like would be:  

- The `DWARFLinker/` root directory contains shared classes and lives in the `dwarflinker` namespace. I think it should probably be `dwarf_linker` but I'm fine with `dwarflinker` if that's too much churn. 
- The `DWARFLinker/Classic` directory containing classes needed for the dsymutil lockstep algorithm. Everything here should live under the `dwarf(_)linker::classic` namespace. 
- The `DWARFLinker/Parallel` directory containing classes needed for the parallel algorithm. Everything here should live under the `dwarf(_)linker::parallel` namespace.


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