[lld-macho] Make writing map file asynchronous

For large applications that write to map files, writing map files can take quite
a bit of time. Sorting the biggest contributors to link times, writing map files
ranks in at 2nd place, with load input files being the biggest contributor of
link times. Avoiding writing map files on the critical path (and having its own
thread) saves ~2-3 seconds when linking chromium framework on a 16-Core
Intel Xeon W.

```
           base            diff            difference (95% CI)
sys_time   1.617 ± 0.034   1.657 ± 0.026   [  +1.5% ..   +3.5%]
user_time  28.536 ± 0.245  28.609 ± 0.180  [  -0.1% ..   +0.7%]
wall_time  23.833 ± 0.271  21.684 ± 0.194  [  -9.5% ..   -8.5%]
samples    31              24
```

Reviewed By: #lld-macho, oontvoo, int3

Differential Revision: https://reviews.llvm.org/D115416