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

[Daniel Rodríguez Troitiño via llvm-commits]

drodriguez wrote:

@johnno1962 do you want to modify the summary with the final spelling of the flag and maybe updating the working for some later results?

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