[5.5][Runtime] Fix crash in protocol conformance checks on KVO artificial subclasses.

tryGetCompleteMetadataNonblocking crashes on artificial subclasses due to the NULL type descriptor. Explicitly check for artificial subclasses in getSuperclassForMaybeIncompleteMetadata and immediately return their Superclass field. Artificial subclasses are always fully initialized so we don't need to do anything special for them.

rdar://72583931
(cherry picked from commit c3aa443)

Author: mikeash

stdlib/public/runtime/ProtocolConformance.cpp

@@ -169,6 +169,14 @@ static MetadataResponse getSuperclassForMaybeIncompleteMetadata(
   if (!classMetadata)
     return {_swift_class_getSuperclass(metadata), MetadataState::Complete};
 
+#if SWIFT_OBJC_INTEROP
+    // Artificial subclasses are not valid type metadata and
+    // tryGetCompleteMetadataNonblocking will crash on them. However, they're
+    // always fully set up, so we can just skip it and fetch the Subclass field.
+    if (classMetadata->isTypeMetadata() && classMetadata->isArtificialSubclass())
+      return {classMetadata->Superclass, MetadataState::Complete};
+#endif
+
   MetadataState metadataState;
   if (knownMetadataState)
     metadataState = *knownMetadataState;

test/Casting/Casts.swift

@@ -954,4 +954,14 @@ CastsTests.test("Recursive AnyHashable") {
   expectEqual(s.x, p)
 }
 
+#if _runtime(_ObjC)
+CastsTests.test("Artificial subclass protocol conformance") {
+  class SwiftClass: NSObject {}
+  let subclass: AnyClass = objc_allocateClassPair(SwiftClass.self,
+                                                  "ArtificialSwiftSubclass", 0)!
+  objc_registerClassPair(subclass)
+  expectFalse(subclass is P.Type)
+}
+#endif
+
 runAllTests()