Add a missing check for the number of children before accessing them.

parentDemangling is going to be added as the first child of the return node, so
we can't construct one unless we have both children.

rdar://77530286
(cherry picked from commit a50c048)

Author: adrian-prantl

include/swift/Remote/MetadataReader.h

@@ -2474,6 +2474,9 @@ class MetadataReader {
     // Use private declaration names for anonymous context references.
     if (parentDemangling->getKind() == Node::Kind::AnonymousContext
         && nameNode->getKind() == Node::Kind::Identifier) {
+      if (parentDemangling->getNumChildren() < 2)
+        return nullptr;
+
       auto privateDeclName =
         dem.createNode(Node::Kind::PrivateDeclName);
       privateDeclName->addChild(parentDemangling->getChild(0), dem);