[NFC] Add @objc to imported ObjC categories

This contains the category name, if there is one.

lib/AST/Decl.cpp

@@ -3707,6 +3707,29 @@ void ValueDecl::setIsObjC(bool value) {
   LazySemanticInfo.isObjC = value;
 }
 
+Identifier ExtensionDecl::getObjCCategoryName() const {
+  // If there's an @objc attribute, it's authoritative. (ClangImporter
+  // attaches one automatically.)
+  if (auto objcAttr = getAttrs().getAttribute<ObjCAttr>(/*AllowInvalid*/true)) {
+    if (objcAttr->hasName() && objcAttr->getName()->getNumArgs() == 0)
+      return objcAttr->getName()->getSimpleName();
+
+    return Identifier();
+  }
+
+  // Fall back to @_objcImplementation attribute.
+  if (auto attr =
+       getAttrs().getAttribute<ObjCImplementationAttr>(/*AllowInvalid=*/true)) {
+    if (!attr->isCategoryNameInvalid())
+      return attr->CategoryName;
+
+    return Identifier();
+  }
+
+  // Not a category, evidently.
+  return Identifier();
+}
+
 bool ValueDecl::isSemanticallyFinal() const {
   // Actor types are semantically final.
   if (auto classDecl = dyn_cast<ClassDecl>(this)) {

lib/ClangImporter/ClangImporter.cpp

@@ -5969,32 +5969,6 @@ struct OrderDecls {
 };
 }
 
-Identifier ExtensionDecl::getObjCCategoryName() const {
-  // Could it be an imported category?
-  if (!hasClangNode()) {
-    // Nope, not imported. Is it an @implementation extension?
-    auto attr = getAttrs()
-                    .getAttribute<ObjCImplementationAttr>(/*AllowInvalid=*/true);
-    if (attr && !attr->isCategoryNameInvalid())
-      return attr->CategoryName;
-
-    return Identifier();
-  }
-
-  auto category = dyn_cast<clang::ObjCCategoryDecl>(getClangDecl());
-  if (!category)
-    // Nope, not a category.
-    return Identifier();
-
-  // We'll look for an implementation with this category name.
-  auto clangCategoryName = category->getName();
-  if (clangCategoryName.empty())
-    // Class extension (has an empty name).
-    return Identifier();
-  
-  return getASTContext().getIdentifier(clangCategoryName);
-}
-
 static ObjCInterfaceAndImplementation
 constructResult(const llvm::TinyPtrVector<Decl *> &interfaces,
                 llvm::TinyPtrVector<Decl *> &impls,

lib/ClangImporter/ImportDecl.cpp

@@ -4132,14 +4132,16 @@ namespace {
     /// selector.
     ///
     /// The importer should use this rather than adding the attribute directly.
-    void addObjCAttribute(ValueDecl *decl, std::optional<ObjCSelector> name) {
+    void addObjCAttribute(Decl *decl, std::optional<ObjCSelector> name) {
       auto &ctx = Impl.SwiftContext;
       if (name) {
         decl->getAttrs().add(ObjCAttr::create(ctx, name,
                                               /*implicitName=*/true));
       }
-      decl->setIsObjC(true);
-      decl->setIsDynamic(true);
+      if (auto VD = dyn_cast<ValueDecl>(decl)) {
+        VD->setIsObjC(true);
+        VD->setIsDynamic(true);
+      }
 
       // If the declaration we attached the 'objc' attribute to is within a
       // type, record it in the type.
@@ -4157,7 +4159,7 @@ namespace {
     /// selector.
     ///
     /// The importer should use this rather than adding the attribute directly.
-    void addObjCAttribute(ValueDecl *decl, Identifier name) {
+    void addObjCAttribute(Decl *decl, Identifier name) {
       addObjCAttribute(decl, ObjCSelector(Impl.SwiftContext, 0, name));
     }
 
@@ -4778,6 +4780,11 @@ namespace {
                                               objcClass->getDeclaredType());
       Impl.SwiftContext.evaluator.cacheOutput(ExtendedNominalRequest{result},
                                               std::move(objcClass));
+      
+      Identifier categoryName;
+      if (!decl->getName().empty())
+        categoryName = Impl.SwiftContext.getIdentifier(decl->getName());
+      addObjCAttribute(result, categoryName);
 
       // Create the extension declaration and record it.
       objcClass->addExtension(result);