[Runtime] Extend Metadata::dump for existential type metadata.

Dump whether it's class bounded, the protocols it contains, and the superclass constraint if there is one.

stdlib/public/runtime/Metadata.cpp

@@ -3958,6 +3958,24 @@ void Metadata::dump() const {
     printf("Labels: %s.\n", tuple->Labels);
   }
 
+  if (auto *existential = dyn_cast<ExistentialTypeMetadata>(this)) {
+    printf("Is class bounded: %s.\n",
+           existential->isClassBounded() ? "true" : "false");
+    auto protocols = existential->getProtocols();
+    bool first = true;
+    printf("Protocols: ");
+    for (auto protocol : protocols) {
+      if (!first)
+        printf(" & ");
+      printf("%s", protocol.getName());
+      first = false;
+    }
+    if (auto *superclass = existential->getSuperclassConstraint())
+      if (auto *contextDescriptor = superclass->getTypeContextDescriptor())
+        printf("Superclass constraint: %s.\n", contextDescriptor->Name.get());
+    printf("\n");
+  }
+
   printf("Generic Args: %p.\n", getGenericArgs());
 
 #if SWIFT_OBJC_INTEROP