Revert r305678: [driver][macOS] Pick the system version for the

deployment target if the SDK is newer than the system

This commit also reverts follow-up commits r305680 and r305685 that have
buildbot fixes.

The change in r305678 wasn't correct because it relied on
`llvm::sys::getProcessTriple`, which uses a pre-configured OS version. We should
lookup the actual macOS version of the system on which the compiler is running.

llvm-svn: 305891