Adopting the Parallel DWARF linker in dsymutil

On Apple platforms, the development experience was designed around making the compile-link-debug cycle as fast as possible. For debugging, that means that rather than processing large amounts of DWARF to link it into the final binary, the linker leaves the debug info in the object files and records a debug map that tells the debugger where to find it. When you're debugging locally, that's all you need. But if you want to archive the debug info for crash reporting or remote debugging, you need a way to produce a self-contained bundle.