[lld-macho][nfc] Canonicalize all pointers to InputSections early on

Having to remember to call `canonical()` all over the place is
error-prone; let's do it in a centralized location instead. It also
appears to improve performance slightly.

             base           diff           difference (95% CI)
  sys_time   0.984 ± 0.009  0.983 ± 0.014  [  -0.8% ..   +0.6%]
  user_time  6.508 ± 0.035  6.475 ± 0.036  [  -0.8% ..   -0.2%]
  wall_time  5.321 ± 0.034  5.300 ± 0.033  [  -0.7% ..   -0.1%]
  samples    36             23

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D112687