AST: Introduce EnumDecl::isCDeclEnum and isCCompatibleEnum

include/swift/AST/Decl.h

@@ -4874,6 +4874,16 @@ public:
     return getAttrs().hasAttribute<IndirectAttr>();
   }
 
+  /// True if the enum is marked with `@cdecl`.
+  bool isCDeclEnum() const {
+    return getAttrs().hasAttribute<CDeclAttr>();
+  }
+
+  /// True if the enum is marked with `@cdecl` or `@objc`.
+  bool isCCompatibleEnum() const {
+    return isCDeclEnum() || isObjC();
+  }
+
   /// True if the enum can be exhaustively switched within \p useDC.
   ///
   /// Note that this property is \e not necessarily true for all children of

lib/AST/ClangTypeConverter.cpp

@@ -654,7 +654,7 @@ clang::QualType ClangTypeConverter::visitEnumType(EnumType *type) {
     return convert(Context.TheEmptyTupleType);
 
   auto ED = type->getDecl();
-  if (!ED->isObjC() && !ED->getAttrs().hasAttribute<CDeclAttr>())
+  if (!ED->isCCompatibleEnum())
     // Can't translate something not marked with @objc or @cdecl.
     return clang::QualType();
 

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -2408,7 +2408,7 @@ private:
   void maybePrintTagKeyword(const TypeDecl *NTD) {
     auto *ED = dyn_cast<EnumDecl>(NTD);
     if (ED && !NTD->hasClangNode()) {
-      if (ED->getAttrs().hasAttribute<CDeclAttr>()) {
+      if (ED->isCDeclEnum()) {
         // We should be able to use the tag macro for all printed enums but
         // for now restrict it to @cdecl to guard it behind the feature flag.
         os << "SWIFT_ENUM_TAG ";

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -538,8 +538,7 @@ public:
   }
 
   void forwardDeclare(const EnumDecl *ED) {
-    assert(ED->isObjC() || ED->getAttrs().getAttribute<CDeclAttr>() ||
-           ED->hasClangNode());
+    assert(ED->isCCompatibleEnum() || ED->hasClangNode());
     
     forwardDeclare(ED, [&]{
       if (ED->getASTContext().LangOpts.hasFeature(Feature::CDecl)) {