[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