Previously: "module compiled with Swift 4.1 cannot be imported in Swift
4.1.50" (i.e. following the -swift-version flag)
Now: "module compiled with Swift 4.1 cannot be imported by the Swift
4.2 compiler"
I'm pretty sure this is what I intended to do all along, and I just
messed it up when I originally implemented it. This is especially
important when working with downloadable toolchains, which would say
"module compiled with Swift 4.2 cannot be imported in Swift 4.1.50",
which is not really the problem at all. Now it'll fall back to the
more generic "module file was created by an older version of the
compiler" error.
People were getting confused when it said "module compiled with Swift
3.1 cannot be imported into Swift 4.0" when they were passing
"-swift-version 3".
rdar://problem/32187112
...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
