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

[via llvm-commits]

avl-llvm wrote:

addressed comments(renamed directories Apple->Classic, LLVM->Parallel), renamed "dwarflinker" namespace into "dwarf_linker", renamed "dwarflinker_parallel" namespace into "parallel", added "classic" namespace as a child of "dwarf_linker").

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