Simplify @objc actor back-deploy IR and switch to objc_getRequiredClass.

`objc_getRequiredClass` will produce a fatal error if the class isn't
found, which will prevent a malformed program using back-deployed @objc
actor from launching. Also eliminate the spurious `objc_opt_self`
call, which is unneeded given that we're realizing the metadata.

Thanks to Mike Ash for the review.

Author: DougGregor

include/swift/Runtime/RuntimeFunctions.def

@@ -1390,6 +1390,10 @@ FUNCTION(ObjCGetClass, objc_getClass, C_CC,  AlwaysAvailable,
          RETURNS(ObjCClassPtrTy),
          ARGS(Int8PtrTy),
          ATTRS(NoUnwind))
+FUNCTION(ObjCGetRequiredClass, objc_getRequiredClass, C_CC,  AlwaysAvailable,
+         RETURNS(ObjCClassPtrTy),
+         ARGS(Int8PtrTy),
+         ATTRS(NoUnwind))
 FUNCTION(ObjCGetMetaClass, objc_getMetaClass, C_CC, AlwaysAvailable,
          RETURNS(ObjCClassPtrTy),
          ARGS(Int8PtrTy),

lib/IRGen/GenDecl.cpp

@@ -1881,15 +1881,13 @@ void IRGenerator::emitObjCActorsNeedingSuperclassSwizzle() {
   auto swiftNativeNSObjectName =
       IGM->getAddrOfGlobalString("SwiftNativeNSObject");
   auto swiftNativeNSObjectClass = RegisterIGF.Builder.CreateCall(
-      RegisterIGF.IGM.getLookUpClassFn(), swiftNativeNSObjectName);
+      RegisterIGF.IGM.getObjCGetRequiredClassFn(), swiftNativeNSObjectName);
 
   for (ClassDecl *CD : ObjCActorsNeedingSuperclassSwizzle) {
     // The @objc actor class.
     llvm::Value *classRef = RegisterIGF.emitTypeMetadataRef(
         CD->getDeclaredInterfaceType()->getCanonicalType());
     classRef = RegisterIGF.Builder.CreateBitCast(classRef, IGM->ObjCClassPtrTy);
-    classRef = RegisterIGF.Builder.CreateCall(
-        IGM->getFixedClassInitializationFn(), classRef);
 
     // Set its superclass to SwiftNativeNSObject.
     RegisterIGF.Builder.CreateCall(
@@ -1900,8 +1898,8 @@ void IRGenerator::emitObjCActorsNeedingSuperclassSwizzle() {
 
   // Add the registration function as a static initializer. We use a priority
   // slightly lower than used for C++ global constructors, so that the code is
-  // executed before C++ global constructors (in case someone uses archives
-  // from a C++ global constructor).
+  // executed before C++ global constructors (in case someone manages to access
+  // an @objc actor from a global constructor).
   llvm::appendToGlobalCtors(IGM->Module, RegisterFn, 60000, nullptr);
 }
 

test/IRGen/actor_class_objc_backdeploy.swift

@@ -51,10 +51,9 @@ import Foundation
 // CHECK: swift_defaultActor_destroy
 
 // CHECK-LABEL: define private void @_swift_objc_actor_initialization()
-// CHECK: [[SWIFT_NATIVE_NSOBJECT_CLASS:%.*]]  = call %objc_class* @objc_lookUpClass(i8* getelementptr inbounds ([20 x i8], [20 x i8]* [[SWIFT_NATIVE_NSOBJECT_NAME]]
+// CHECK: [[SWIFT_NATIVE_NSOBJECT_CLASS:%.*]]  = call %objc_class* @objc_getRequiredClass(i8* getelementptr inbounds ([20 x i8], [20 x i8]* [[SWIFT_NATIVE_NSOBJECT_NAME]]
 // CHECK: [[ACTOR_RESPONSE:%.*]] = call swiftcc %swift.metadata_response @"$s16actor_class_objc7MyClassCMa"(
 // CHECK: [[ACTOR_METADATA:%.*]] = extractvalue %swift.metadata_response [[ACTOR_RESPONSE]], 0
-// CHECK: [[ACTOR_CLASS_RAW:%.*]] = bitcast %swift.type* [[ACTOR_METADATA]] to %objc_class*
-// CHECK: [[ACTOR_CLASS:%.*]] = call %objc_class* @objc_opt_self(%objc_class* [[ACTOR_CLASS_RAW]])
+// CHECK: [[ACTOR_CLASS:%.*]] = bitcast %swift.type* [[ACTOR_METADATA]] to %objc_class*
 // CHECK: call %objc_class* @class_setSuperclass(%objc_class* [[ACTOR_CLASS]], %objc_class* [[SWIFT_NATIVE_NSOBJECT_CLASS]])