[driver][macOS] Pick the system version for the deployment target

if the SDK is newer than the system

This commit improves the driver by making sure that it picks the system version
for the deployment target when the version of the macOS SDK is newer than the
system version.

rdar://29449467

Differential Revision: https://reviews.llvm.org/D34175


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305678 91177308-0d34-0410-b5e6-96231b3b80d8