Support unavailable and obsoleted attributes on extensions

Members of an extension inherit the availability of the extension.
This was previously supported for introduced and deprecated, but not
unavailable and obsoleted.

rdar://problem/50949936

Author: xymus

lib/AST/Attr.cpp

@@ -1062,7 +1062,14 @@ AvailableVersionComparison AvailableAttr::getVersionAvailability(
 
 const AvailableAttr *AvailableAttr::isUnavailable(const Decl *D) {
   ASTContext &ctx = D->getASTContext();
-  return D->getAttrs().getUnavailable(ctx);
+  if (auto attr = D->getAttrs().getUnavailable(ctx))
+    return attr;
+
+  // If D is an extension member, check if the extension is unavailable.
+  if (auto ext = dyn_cast<ExtensionDecl>(D->getDeclContext()))
+    return AvailableAttr::isUnavailable(ext);
+
+  return nullptr;
 }
 
 SpecializeAttr::SpecializeAttr(SourceLoc atLoc, SourceRange range,