[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

@@ -2856,6 +2856,23 @@ namespace {
         return nullptr;
 
       auto Loc = Impl.importSourceLoc(Decl->getLocation());
+
+      if (importer::isUnavailableInSwift(Decl, nullptr, true) &&
+          Impl.SwiftContext.LangOpts.EnableCXXInterop) {
+        ASTContext &ctx = Impl.SwiftContext;
+        auto structDecl = Impl.createDeclWithClangNode<StructDecl>(
+            Decl, AccessLevel::Public, Loc, Name, Loc, None, nullptr, DC);
+
+        makeStructRawValued(Impl, structDecl, SwiftType,
+                            {KnownProtocolKind::OptionSet});
+        auto selfType = structDecl->getDeclaredInterfaceType();
+        addSynthesizedTypealias(structDecl, ctx.Id_Element, selfType);
+        addSynthesizedTypealias(structDecl, ctx.Id_ArrayLiteralElement,
+                                selfType);
+        // Impl.DeclsWithSuperfluousTypedefs.insert(decl);
+        return structDecl;
+      }
+
       auto Result = Impl.createDeclWithClangNode<TypeAliasDecl>(Decl,
                                       AccessLevel::Public,
                                       Impl.importSourceLoc(Decl->getBeginLoc()),

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