[driver][darwin] Refactor the target selection code, NFC

The simulator variant of Darwin's platforms is removed in favor of a new
environment field.
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
environment.

rdar://35813850

Differential Revision: https://reviews.llvm.org/D41035

llvm-svn: 320235