Merge pull request #38319 from mikeash/fix-protocol-conformance-pure-objc-classes-5.5

[5.5][Runtime] Fix protocol conformance checks on pure ObjC classes.

Author: DougGregor

stdlib/public/runtime/ProtocolConformance.cpp

@@ -175,6 +175,11 @@ static MetadataResponse getSuperclassForMaybeIncompleteMetadata(
     // 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};
+
+    // Pure ObjC classes are already set up, and the code below will not be
+    // happy with them.
+    if (!classMetadata->isTypeMetadata())
+      return {classMetadata->Superclass, MetadataState::Complete};
 #endif
 
   MetadataState metadataState;