IRGen: Perform completely fragile layout for all classes, not just @objc ancestry

Otherwise, Objective C categories don't work for classes with
resilient fields, such as the value types defined in the Foundation
overlay.

Fixes <rdar://problem/40819319>, <https://bugs.swift.org/browse/SR-7882>.

lib/IRGen/GenClass.cpp

@@ -141,9 +141,9 @@ namespace {
 
     unsigned NumInherited = 0;
 
-    // If the class has @objc ancestry, we lay out resiliently-typed fields
-    // as if they were fragile.
-    bool CompletelyFragileLayout = false;
+    // For now we always lay out resiliently-typed fields as if they
+    // were fragile.
+    bool CompletelyFragileLayout;
 
     // Does the class metadata require dynamic initialization above and
     // beyond what the runtime can automatically achieve?
@@ -171,6 +171,11 @@ namespace {
                        ReferenceCounting refcounting)
       : StructLayoutBuilder(IGM)
     {
+      // Perform fragile layout if Objective-C interop is enabled.
+      CompletelyFragileLayout =
+        (IGM.Context.LangOpts.EnableObjCInterop &&
+         !IGM.IRGen.Opts.EnableClassResilience);
+
       // Start by adding a heap header.
       switch (refcounting) {
       case ReferenceCounting::Native:
@@ -249,10 +254,6 @@ namespace {
         }
 
         if (superclass->hasClangNode()) {
-          // Perform fragile layout if the class has @objc ancestry.
-          if (!IGM.IRGen.Opts.EnableClassResilience)
-            CompletelyFragileLayout = true;
-
           // If the superclass was imported from Objective-C, its size is
           // not known at compile time. However, since the field offset
           // vector only stores offsets of stored properties defined in