[Darwin] Cleanup code via improved GetMacosAlignedVersion()

Checking the OS version via `GetMacosAlignedVersion()` now works in
simulators [1].  Let's use it to simplify `DyldNeedsEnvVariable()`.

[1] 3fb0de82

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D81197