[lld] [lld-macho] Parallelize ObjFile::sourceFile() during STABS emission. (PR #222087)

[Liza Burakova via llvm-commits]

liza371 wrote:

Just a note: my machine was apparently too noisy when I did the benchmark tests for this PR, so I redid them on `mac_debug_component_sym_2` and here are the updated results

```
Baseline 1
  Time (mean ± σ):      7.907 s ±  0.180 s    [User: 7.013 s, System: 2.350 s]
  Range (min … max):    7.722 s …  8.430 s    20 runs

Optimized
  Time (mean ± σ):      7.572 s ±  0.137 s    [User: 7.294 s, System: 2.533 s]
  Range (min … max):    7.394 s …  7.844 s    20 runs

Baseline 2
  Time (mean ± σ):      7.885 s ±  0.187 s    [User: 7.046 s, System: 2.415 s]
  Range (min … max):    7.665 s …  8.315 s    20 runs
```

https://github.com/llvm/llvm-project/pull/222087