Introduce `MessageDescriptor.IsMapType` in C# reflection to make it easier to tell whether a given message was generated by protoc for a map field.

PiperOrigin-RevId: 628087118

csharp/src/Google.Protobuf.Test/Reflection/DescriptorsTest.cs

@@ -204,6 +204,14 @@ namespace Google.Protobuf.Reflection
             TestDescriptorToProto(messageType.ToProto, messageType.Proto);
         }
 
+        [Test]
+        public void MessageDescriptor_IsMapEntry()
+        {
+            var testMapMessage = TestMap.Descriptor;
+            Assert.False(testMapMessage.IsMapEntry);
+            Assert.True(testMapMessage.Fields[1].MessageType.IsMapEntry);
+        }
+
         [Test]
         public void FieldDescriptor_GeneratedCode()
         {

csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs

@@ -242,7 +242,7 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// Returns <c>true</c> if this field is a map field; <c>false</c> otherwise.
         /// </summary>
-        public bool IsMap => FieldType == FieldType.Message && messageType.Proto.Options != null && messageType.Proto.Options.MapEntry;
+        public bool IsMap => FieldType == FieldType.Message && messageType.IsMapEntry;
 
         /// <summary>
         /// Returns <c>true</c> if this field is a packed, repeated field; <c>false</c> otherwise.

csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs

@@ -202,6 +202,12 @@ namespace Google.Protobuf.Reflection
         /// </summary>
         internal bool IsWrapperType => File.Package == "google.protobuf" && File.Name == "google/protobuf/wrappers.proto";
 
+        /// <summary>
+        /// Returns whether this message was synthetically-created to store key/value pairs in a
+        /// map field.
+        /// </summary>
+        public bool IsMapEntry => Proto.Options?.MapEntry == true;
+
         /// <value>
         /// If this is a nested type, get the outer descriptor, otherwise null.
         /// </value>