[c++-interop] Adding options set struct to go with the typedef's enum bit set.

This does not work yet, but it is a proof of concept for now.

Author: plotfi

lib/ClangImporter/ImportDecl.cpp

@@ -2856,14 +2856,46 @@ namespace {
         return nullptr;
 
       auto Loc = Impl.importSourceLoc(Decl->getLocation());
-      auto Result = Impl.createDeclWithClangNode<TypeAliasDecl>(Decl,
-                                      AccessLevel::Public,
-                                      Impl.importSourceLoc(Decl->getBeginLoc()),
-                                      SourceLoc(), Name,
-                                      Loc,
-                                      /*genericparams*/nullptr, DC);
-
-      Result->setUnderlyingType(SwiftType);
+
+      swift::Decl *Result = nullptr;
+      if (importer::isUnavailableInSwift(Decl, nullptr, true) &&
+          Impl.SwiftContext.LangOpts.EnableCXXInterop) {
+        ASTContext &ctx = Impl.SwiftContext;
+
+        // Compute the underlying type.
+        clang::QualType ClangType = Decl->getUnderlyingType();
+        auto underlyingType = Impl.importTypeIgnoreIUO(
+            ClangType, ImportTypeKind::Enum,
+            ImportDiagnosticAdder(Impl, Decl, Decl->getLocation()),
+            isInSystemModule(DC), Bridgeability::None, ImportTypeAttrs());
+        if (!underlyingType)
+          return nullptr;
+
+        auto structDecl = Impl.createDeclWithClangNode<StructDecl>(
+            Decl, AccessLevel::Public, Loc, Name, Loc, None, nullptr, DC);
+
+        addSynthesizedProtocolAttrs(Impl, structDecl,
+                                    {KnownProtocolKind::Sendable},
+                                    /*isUnchecked=*/true);
+
+        makeStructRawValued(Impl, structDecl, underlyingType,
+                            {KnownProtocolKind::OptionSet});
+        auto selfType = structDecl->getDeclaredInterfaceType();
+        addSynthesizedTypealias(structDecl, ctx.Id_Element, selfType);
+        addSynthesizedTypealias(structDecl, ctx.Id_ArrayLiteralElement,
+                                selfType);
+        Impl.DeclsWithSuperfluousTypedefs.insert(Decl);
+
+        Result = structDecl;
+      } else {
+        auto TypeAliasResult = Impl.createDeclWithClangNode<TypeAliasDecl>(
+            Decl, AccessLevel::Public,
+            Impl.importSourceLoc(Decl->getBeginLoc()), SourceLoc(), Name, Loc,
+            /*genericparams*/ nullptr, DC);
+
+        TypeAliasResult->setUnderlyingType(SwiftType);
+        Result = TypeAliasResult;
+      }
 
       // Make Objective-C's 'id' unavailable.
       if (Impl.SwiftContext.LangOpts.EnableObjCInterop && isObjCId(Decl)) {

test/Interop/Cxx/enum/c-enums-withOptions-omit.swift

@@ -3,6 +3,13 @@
 
 import CenumsWithOptionsOmit
 
+// CHECK: struct NSEnumerationOptions : OptionSet {
+// CHECK-NEXT:   init(rawValue: Int32)
+// CHECK-NEXT:   let rawValue: Int32
+// CHECK-NEXT:   typealias RawValue = Int32
+// CHECK-NEXT:   typealias Element = NSEnumerationOptions
+// CHECK-NEXT:   typealias ArrayLiteralElement = NSEnumerationOptions
+
 // CHECK: class NSSet {
 // CHECK-NEXT: class func enumerateObjects(options
 // CHECK-NEXT: func enumerateObjects(options