[lld] [lld][MachO]Multi-threaded i/o. Twice as fast linking a large project. (PR #147134)

[John Holdsworth via llvm-commits]

johnno1962 wrote:

> I would like to hear more feedback about the structure of the code. I would certainly do things differently, but I have not tried to solve the problem, so I might be thinking other structure is possible when you already have looked at those options. Sorry if I am proposing something you tested and didn't work.
I think the structure is fine even after these changes which are still a proof of concept. If this PR merges you may want to make another pass over the code to structure it how you like. If you have any specific changes you want to make now, for example to the `parallelFor` loop, let me know and I can benchmark them.

https://github.com/llvm/llvm-project/pull/147134