[lld] [llvm] [DTLTO][ELF][COFF] Add archive support for DTLTO. (PR #157043)

[Teresa Johnson via llvm-commits]

https://github.com/teresajohnson approved this pull request.

Thanks looks good just some minor cleanups noted below (isThinArchive can become a private member func, make member funcs/vars private where possible, and add documenting comments to the declarations). Otherwise lgtm

https://github.com/llvm/llvm-project/pull/157043