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
Alex Lorenz