[ClangImporter] Import unavailable decls as unavailable

Author: xymus

lib/ClangImporter/ImportDecl.cpp

@@ -1817,18 +1817,27 @@ static void applyAvailableAttribute(Decl *decl, AvailabilityContext &info,
   if (info.isAlwaysAvailable())
     return;
 
+  PlatformAgnosticAvailabilityKind platformAgnosticAvailability;
+  llvm::VersionTuple introducedVersion;
+  if (info.isKnownUnreachable()) {
+    platformAgnosticAvailability = PlatformAgnosticAvailabilityKind::Unavailable;
+  } else {
+    platformAgnosticAvailability = PlatformAgnosticAvailabilityKind::None;
+    introducedVersion = info.getOSVersion().getLowerEndpoint();
+  }
+
   llvm::VersionTuple noVersion;
   auto AvAttr = new (C) AvailableAttr(SourceLoc(), SourceRange(),
                                       targetPlatform(C.LangOpts),
                                       /*Message=*/StringRef(),
                                       /*Rename=*/StringRef(),
-                                      info.getOSVersion().getLowerEndpoint(),
+                                      introducedVersion,
                                       /*IntroducedRange*/SourceRange(),
                                       /*Deprecated=*/noVersion,
                                       /*DeprecatedRange*/SourceRange(),
                                       /*Obsoleted=*/noVersion,
                                       /*ObsoletedRange*/SourceRange(),
-                                      PlatformAgnosticAvailabilityKind::None,
+                                      platformAgnosticAvailability,
                                       /*Implicit=*/false);
 
   decl->getAttrs().add(AvAttr);