Mark unimported attribute item as "expanded" when in attribute context

src/Features/Core/Portable/Completion/CompletionItemFlags.cs

@@ -13,8 +13,8 @@ internal enum CompletionItemFlags
 
         /// <summary>
         /// Indicates this <see cref="CompletionItem"/> is cached and reused across completion sessions. 
-        /// This might be used by completion system for things like deciding whether it can safaly cache and reuse
-        /// other data correspodning to this item.
+        /// This might be used by completion system for things like deciding whether it can safely cache and reuse
+        /// other data corresponding to this item.
         ///
         /// TODO: Revisit the approach we used for caching VS items.
         ///       https://github.com/dotnet/roslyn/issues/35160

src/Features/Core/Portable/Completion/Providers/ImportCompletionProvider/AbstractTypeImportCompletionService.CacheEntry.cs

@@ -87,7 +87,7 @@ static CompletionItem GetAppropriateAttributeItem(CompletionItem attributeItem,
                     if (attributeItem.DisplayText.TryGetWithoutAttributeSuffix(isCaseSensitive: isCaseSensitive, out var attributeNameWithoutSuffix))
                     {
                         // We don't want to cache this item.
-                        return ImportCompletionItem.CreateAttributeItemWithoutSuffix(attributeItem, attributeNameWithoutSuffix);
+                        return ImportCompletionItem.CreateAttributeItemWithoutSuffix(attributeItem, attributeNameWithoutSuffix, CompletionItemFlags.Expanded);
                     }
 
                     return attributeItem;

src/Features/Core/Portable/Completion/Providers/ImportCompletionProvider/ImportCompletionItem.cs

@@ -68,7 +68,7 @@ public static CompletionItem Create(string name, int arity, string containingNam
             return item;
         }
 
-        public static CompletionItem CreateAttributeItemWithoutSuffix(CompletionItem attributeItem, string attributeNameWithoutSuffix)
+        public static CompletionItem CreateAttributeItemWithoutSuffix(CompletionItem attributeItem, string attributeNameWithoutSuffix, CompletionItemFlags flags)
         {
             Debug.Assert(!attributeItem.Properties.ContainsKey(AttributeFullName));
 
@@ -78,7 +78,7 @@ public static CompletionItem CreateAttributeItemWithoutSuffix(CompletionItem att
             var sortTextBuilder = PooledStringBuilder.GetInstance();
             sortTextBuilder.Builder.AppendFormat(SortTextFormat, attributeNameWithoutSuffix, attributeItem.InlineDescription);
 
-            return CompletionItem.Create(
+            var item = CompletionItem.Create(
                  displayText: attributeNameWithoutSuffix,
                  sortText: sortTextBuilder.ToStringAndFree(),
                  properties: newProperties,
@@ -87,6 +87,9 @@ public static CompletionItem CreateAttributeItemWithoutSuffix(CompletionItem att
                  displayTextPrefix: attributeItem.DisplayTextPrefix,
                  displayTextSuffix: attributeItem.DisplayTextSuffix,
                  inlineDescription: attributeItem.InlineDescription);
+
+            item.Flags = flags;
+            return item;
         }
 
         public static CompletionItem CreateItemWithGenericDisplaySuffix(CompletionItem item, string genericTypeSuffix)