[clang] [llvm] [LLVM] Add GNU make jobserver support (PR #145131)

[Andrew Ng via llvm-commits]

nga888 wrote:

Sorry that I haven't commented earlier but I have only just got around to looking at this in more detail. Although, the parallel functions for `Parallel.cpp` already "batch", I would still be a bit concerned about the overhead for obtaining the job slot. Perhaps like in `ThreadPool.cpp` it should hold onto the job slot whilst there are jobs to be processed? I also see that an exponential back-off is used in `ThreadPool.cpp`. Could this be applicable also to `Parallel.cpp`?

https://github.com/llvm/llvm-project/pull/145131