Exclude @available(swift) from PlatformKind::none Availability inference.

Author: graydon

lib/AST/Availability.cpp

@@ -128,7 +128,8 @@ AvailabilityInference::annotatedAvailableRange(const Decl *D, ASTContext &Ctx) {
   for (auto Attr : D->getAttrs()) {
     auto *AvailAttr = dyn_cast<AvailableAttr>(Attr);
     if (AvailAttr == nullptr || !AvailAttr->Introduced.hasValue() ||
-        !AvailAttr->isActivePlatform(Ctx)) {
+        !AvailAttr->isActivePlatform(Ctx) ||
+        AvailAttr->isLanguageVersionSpecific()) {
       continue;
     }