System.CodeDom.MemberAttributes enum is used as a flag but doesn't have the [Flags] attribute

When defining a `CodeMember*` object, the `Attributes` can be assigned using a bitwise combination of MemberAttributes. While this works and produces the expected code output, the lack of the `[Flags]` enum causes some IDEs, like Rider, to emit a warning that the enum is being used as if it were a flag enum without having the attribute.

To resolve, the `[Flags]` attribute should be added to [`MemberAttributes.cs`](https://github.com/dotnet/runtime/blob/main/src/libraries/System.CodeDom/src/System/CodeDom/MemberAttributes.cs). No other changes are necessary,