[driver][darwin] Refactor the target selection code, NFC
The simulator variant of Darwin's platforms is removed in favor of a new
The code that selects the platform and the version is split into 4 different
functions instead of being all in one function.
This is an NFC commit, although it slightly improves the
"invalid version number" diagnostic by displaying the environment variable
instead of -m<os>-version-min if the OS version was derived from the
Differential Revision: https://reviews.llvm.org/D41035