[lld/mac] Tweak estimateStubsInRangeVA a bit

- Move a few variables closer to their uses, remove some completely
  (no behavior change)
- Add some comments
- Make maxPotentialThunks include calls to stubs. It's possible that
  an earlier call to a stub late in the stub table will need a thunk,
  and that inserted thunk could push a stub earlier in the stub table
  out of range. This is unlikely to happen, but usually there are
  way fewer stub calls than non-stub calls, so if we're doing a
  conservative approximation here we might as well do it correctly.
  (For chromium's unit_tests target, 134421/242639 stub calls are
  direct calls without this change, compared to 134408/242639 with
  this change)

No real, meaningful behavior difference.

Differential Revision: https://reviews.llvm.org/D108924