[clang][darwin] Add support for the -mtargetos= option to the driver

The new -mtargetos= option is a replacement for the existing, OS-specific options
like -miphoneos-version-min=. This allows us to introduce support for new darwin OSes
easier as they won't require the use of a new option. The older options will be
deprecated and the use of the new option will be encouraged instead.

Differential Revision: https://reviews.llvm.org/D106316