Issue Description:

swift(>=x.y) is a special compilation condition that controls whether part of a file is even parsed, because the language may change between versions. Because of this, there's a restriction that it can't be combined with other compilation conditions, which still check that the inactive block of the #if parses correctly. However, there's nothing wrong with `#if !swift(>=4.1)`, and therefore it would be reasonable to support spelling that directly, `#if swift(<4.1)`.

(Note that it would not make sense to check for swift(>x.y) or swift(<=x.y), since that makes a statement about future releases rather than present releases. For example, even when 4.1.0 is officially released, that wouldn't preclude another release from the 4.0 branch, 4.0.4.)