Specifically mention version mismatches for non-subclassable classes.

That is, if a Swift 4 class has members that cannot be loaded in Swift
3 mode and that's why it can't be subclassed, mention that in the
diagnostic.

include/swift/Serialization/SerializedModuleLoader.h

@@ -120,6 +120,10 @@ class SerializedASTFile final : public LoadedFile {
 public:
   bool isSIB() const { return IsSIB; }
 
+  /// Returns the language version that was used to compile the contents of this
+  /// file.
+  const version::Version &getLanguageVersionBuiltWith() const;
+
   virtual bool isSystemModule() const override;
 
   virtual void lookupValue(ModuleDecl::AccessPathTy accessPath,