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

[Ellis Hoag via llvm-commits]

https://github.com/ellishg commented:

I don't see anything obviously wrong with this current revision, but concurrency is hard so I'll give others more time to review

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