Issue Description:

The recently-added -swift-version N flag in SR-2582 only affects the #if swift(>=N) compiler-control statements, which is not sufficient for shipping a library with parts that are unconditionally compiled but conditionally available. For the latter, we need to support something like @available(swift N) similar to the platform availability tests.

The exact semantics are a bit subtle; as far as I can tell, swift-version unavailability should override platform availability, but swift-version availability should not override platform unavailability. That is, swift-version availability should be considered in conjunction, not disjunction, with platform availability. We don't exactly have a way to write availability conjuncts at the moment, so as an initial implementation we might just prohibit combining `@available(swift N)` with any other platform-availability spec in the same attribute argument list.

Other suggestions welcome!