[lld-macho][nfc] Use llvm::enumerate + destructuring in more places

I love C++17!

chromium_framework_less_dwarf on my 16-core Mac Pro shows no stat sig change in wall time but a slight decrease in user time:

```
           base           diff           difference (95% CI)
sys_time   1.759 ± 0.037  1.761 ± 0.033  [  -0.9% ..   +1.1%]
user_time  4.920 ± 0.043  4.886 ± 0.051  [  -1.2% ..   -0.2%]
wall_time  5.950 ± 0.117  5.900 ± 0.116  [  -1.8% ..   +0.2%]
samples    26             37
```

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D136518