[Demangle] Check for old-style mangling in getObjCClassByMangledName

This caused an issue where the runtime was unable to find subclasses of
resilient subclasses of NSObject until they were first registered by their
sugared name with NSClassFromString or were instantiated directly.

rdar://48892003

Author: Harlan Haskins

stdlib/public/runtime/MetadataLookup.cpp

@@ -1422,9 +1422,28 @@ static objc_hook_getClass OldGetClassHook;
 static BOOL
 getObjCClassByMangledName(const char * _Nonnull typeName,
                           Class _Nullable * _Nonnull outClass) {
-  auto metadata = swift_getTypeByMangledNameInEnvironment(typeName, strlen(typeName),
-                                             /* no substitutions */
-                                             nullptr, nullptr);
+  // Demangle old-style class and protocol names, which are still used in the
+  // ObjC metadata.
+  StringRef typeStr(typeName);
+  const Metadata *metadata = nullptr;
+  if (typeStr.startswith("_Tt")) {
+    Demangler demangler;
+    auto node = demangler.demangleSymbol(typeName);
+    if (!node)
+      return NO;
+    metadata = swift_getTypeByMangledNode(
+      MetadataState::Complete, demangler, node,
+      /* no substitutions */
+      [&](unsigned depth, unsigned index) {
+        return nullptr;
+      },
+      [&](const Metadata *type, unsigned index) {
+        return nullptr;
+      }).getMetadata();
+  } else {
+    metadata = swift_getTypeByMangledNameInEnvironment(
+      typeStr.data(), typeStr.size(), /* no substitutions */ nullptr, nullptr);
+  }
   if (metadata) {
     auto objcClass =
       reinterpret_cast<Class>(