Support resilient stored properties in objcImpl

When an @objc @implementation class requires the use of `ClassMetadataStrategy::Update` because some of its stored properties do not have fixed sizes, we adjust the direct field offsets during class realization by emitting a custom metadata update function which calls a new entry point in the Swift runtime. That entry point adjusts field offsets like `swift_updateClassMetadata2()`, but it only assumes that the class has Objective-C metadata, not Swift metadata.

This commit introduces an alternative mechanism which does the same thing without using any Swift-only metadata. It’s a rough implementation with important limitations:

• We’re currently using the field offset vector, which means that field offsets are being emitted into @objc @implementation classes; these will be removed.
• The new Swift runtime entry point duplicates a lot of `swift_updateClassMetadata2()`’s implementation; it will be refactored into something much smaller and more compact.
• Availability bounds for this feature have not yet been implemented.

Future commits in this PR will correct these issues.

include/swift/Runtime/RuntimeFunctions.def

@@ -1437,6 +1437,21 @@ FUNCTION(UpdateClassMetadata2,
          EFFECT(MetaData),
          UNKNOWN_MEMEFFECTS)
 
+// objc_class *swift_updatePureObjCClassMetadata(objc_class *self,
+//                                               ClassLayoutFlags flags,
+//                                               size_t numFields,
+//                                               TypeLayout * const *fieldTypes,
+//                                               size_t *fieldOffsets);
+FUNCTION(UpdatePureObjCClassMetadata,
+         swift_updatePureObjCClassMetadata, SwiftCC, AlwaysAvailable,
+         RETURNS(ObjCClassPtrTy),
+         ARGS(ObjCClassPtrTy, SizeTy, SizeTy,
+              Int8PtrPtrTy->getPointerTo(),
+              SizeTy->getPointerTo()),
+         ATTRS(NoUnwind),
+         EFFECT(MetaData),
+         UNKNOWN_MEMEFFECTS)
+
 // void *swift_lookUpClassMethod(Metadata *metadata,
 //                               ClassDescriptor *description,
 //                               MethodDescriptor *method);