看起来您可以通过使用SchemaFilter<TFilter>选项方法使SwaggerGen自动添加x-ms-enum扩展。
在Startup.cs中进行Swagger注册

// Register the Swagger generator, defining one or more Swagger documents
        services.AddSwaggerGen(c =>
        {
            c.SwaggerDoc("v1", new Info { Title = "Car API", Version = "v1" });
            c.AddSecurityDefinition("Bearer", new ApiKeyScheme { In = "header", Description = "description", Name = "Authorization", Type = "apiKey" });
            c.ApplyFiltersToAllSchemas();//ignore deprecation warning
            c.SchemaFilter<EnumFilter>();
            c.DescribeAllEnumsAsStrings();
        });

然后使用以下代码定义EnumFilter类

public class EnumFilter : ISchemaFilter
{
    public void Apply(Schema model, SchemaFilterContext context)
    {
        if (model == null)
            throw new ArgumentNullException("model");

        if (context == null)
            throw new ArgumentNullException("context");

        if (context.SystemType.IsEnum)
            model.Extensions.Add("x-ms-enum", new
            {
                name = context.SystemType.Name,
                modelAsString = false
            });
    }
}

找了几个小时才找到可空枚举的解决方案！

public class EnumFilter : ISchemaFilter
{

    public void Apply(Schema model, SchemaRegistry schemaRegistry, Type type)
    {
        if (model == null)
        {
            throw new ArgumentNullException("model");
        }

        if (schemaRegistry == null)
        {
            throw new ArgumentNullException("schemaRegistry");
        }

        if (IsEnum(type, out var enumName))
        {
            model.vendorExtensions.Add("x-ms-enum", new
                                              {
                                                  name = enumName ?? type.Name,
                                                  modelAsString = false
                                              });

        }
    }

    public static bool IsEnum(Type t, out string enumName)
    {
        if (t.IsEnum)
        {
            enumName = t.Name;
            return true;
        }
        Type u = Nullable.GetUnderlyingType(t);
        enumName = u?.Name;
        return (u != null) && u.IsEnum;
    }
}

未来的校对解决方案，进一步到@richard-mcdaniel解决方案，一旦未完成的enum values issue on the autorest repo得到了修复，这里的enums部分的值就到了EnumFilter:ISchemaFilter
在Startup.cs中进行Swagger注册

// Register the Swagger generator, defining one or more Swagger documents
        services.AddSwaggerGen(c =>
        {
            c.SwaggerDoc("v1", new Info { Title = "Car API", Version = "v1" });
            c.AddSecurityDefinition("Bearer", new ApiKeyScheme { In = "header", Description = "description", Name = "Authorization", Type = "apiKey" });
            c.ApplyFiltersToAllSchemas();//ignore deprecation warning
            c.SchemaFilter(() => new EnumTypeSchemaFilter(false));
            c.DescribeAllEnumsAsStrings();
        });

然后使用以下代码定义EnumFilter类：

using Swashbuckle.Swagger;
using System;

namespace Swashbuckle.AutoRestExtensions
{
    public class EnumTypeSchemaFilter : ISchemaFilter
    {
        public EnumTypeSchemaFilter()
        {
        }

        public EnumTypeSchemaFilter(bool modelAsString)
        {
            _modelAsString = modelAsString;
        }

        public void Apply(Schema schema, SchemaRegistry schemaRegistry, Type type)
        {
            type = Nullable.GetUnderlyingType(type) ?? type;
            System.Diagnostics.Debug.WriteLine(type.FullName);
            if (type.IsEnum)
            {
                // Add enum type information once
                if (schema.vendorExtensions.ContainsKey("x-ms-enum")) return;

                if (_modelAsString)
                {
                    schema.vendorExtensions.Add("x-ms-enum", new
                    {
                        name = type.Name,
                        modelAsString = _modelAsString
                    });
                }
                else
                {
                    var valuesList = new System.Collections.Generic.List<object>();
                    foreach (var fieldInfo in type.GetFields())
                    {
                        if (fieldInfo.FieldType.IsEnum)
                        {
                            var fName = fieldInfo.Name;
                            var fValue = (int)fieldInfo.GetRawConstantValue();
                            valuesList.Add(new { value = fValue, description = fName, name = fName });
                        }
                    }
                    schema.vendorExtensions.Add("x-ms-enum", new
                    {
                        name = type.Name,
                        modelAsString = _modelAsString,
                        values = valuesList
                        //Values:
                        /*
                        accountType:
                          type: string
                          enum:
                          - Standard_LRS
                          - Standard_ZRS
                          - Standard_GRS
                          - Standard_RAGRS
                          - Premium_LRS
                          x-ms-enum:
                            name: AccountType
                            modelAsString: false
                            values:
                            - value: Standard_LRS
                              description: Locally redundant storage.
                              name: StandardLocalRedundancy
                            - value: Standard_ZRS
                              description: Zone-redundant storage.
                            - value: Standard_GRS
                              name: StandardGeoRedundancy
                            - value: Standard_RAGRS
                            - value: Premium_LRS
                        */
                    });
                }
            }
        }

        private readonly bool _modelAsString;
    }
}

我用那套招摇过市的装置修好了

services.AddSwaggerGenNewtonsoftSupport();
services.AddSwaggerGen(x => x.UseInlineDefinitionsForEnums());