Sema: Report non-LE structs as fragile use sites in CheckImplementationOnly

In non-library-evolution mode, gated behind the CheckImplementationOnly
feature flag, consider structs to be a fragile use site by default,
unless marked `@_implementationOnly`. This prevents them to refer to
restricted imports like implementation-only.

Author: xymus

lib/AST/Decl.cpp

@@ -2759,15 +2759,26 @@ bool VarDecl::isLayoutExposedToClients() const {
   if (!parent) return false;
   if (isStatic()) return false;
 
-
+  auto M = getDeclContext()->getParentModule();
   auto nominalAccess =
     parent->getFormalAccessScope(/*useDC=*/nullptr,
                                  /*treatUsableFromInlineAsPublic=*/true);
-  if (!nominalAccess.isPublic()) return false;
 
-  if (!parent->getAttrs().hasAttribute<FrozenAttr>() &&
-      !parent->getAttrs().hasAttribute<FixedLayoutAttr>())
+  // Resilient modules hide layouts by default.
+  if (!getASTContext().LangOpts.hasFeature(Feature::CheckImplementationOnly) ||
+      M->getResilienceStrategy() == ResilienceStrategy::Resilient) {
+    if (!nominalAccess.isPublic())
+      return false;
+
+    if (!parent->getAttrs().hasAttribute<FrozenAttr>() &&
+        !parent->getAttrs().hasAttribute<FixedLayoutAttr>())
     return false;
+  } else {
+    // Non-resilient module: layouts are exposed by default unless marked
+    // otherwise.
+    if (parent->getAttrs().hasAttribute<ImplementationOnlyAttr>())
+      return false;
+  }
 
   if (!hasStorage() &&
       !getAttrs().hasAttribute<LazyAttr>() &&