...with a better message than the generic "older version of the
compiler" one, when we know it's actually a different version of
Swift proper.
This still uses the same internal module version numbers to check
if the module is compatible; the presentation of language versions
is a diagnostic thing only.
Speaking of module version numbers, this deliberately does NOT
increment VERSION_MINOR; it's implemented in a backwards-compatible
way.
This will only work going forwards, of course; all existing modules
don't have a short version string, and I don't feel comfortable
assuming all older modules we might encounter are "Swift 2.2".
rdar://problem/25680392
