Recommit [driver][macOS] Pick the system version for the

deployment target if the SDK is newer than the system

This commit reverts the revert commit r305891. Now the change from r305678
should be correct because `llvm::sys::getProcessTriple` now returns the correct
macOS version of the system after the LLVM change r307372.

Original commit message:

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

llvm-svn: 307388