Without this, clients that don't use a CompilerInstance (like LLDB) won't
have target configuration options available.
Also, move minimum OS checking into the driver. This makes sure the check
happens early (and only once), and in general fits the philosophy of
allowing the frontend to use configurations that might be banned for users.
<rdar://problem/17688913>
Swift SVN r20701
Also, add more tests for the "os" and "arch" target configurations, to exercise likely
build configurations for iOS and the iOS simulator.
Swift SVN r14307
