[Darwin] Improve runtime OS version checks

Use a struct to represent numerical versions instead of encoding release
names in an enumeration. This avoids the need to extend the enumeration
every time there is a new release.

Rename `GetMacosVersion() -> GetMacosAlignedVersion()` to better reflect
how this is used on non-MacOS platforms.

Reviewed By: delcypher

Differential Revision: https://reviews.llvm.org/D79970