[system.error.syn] Index is_error_condition_enum_v and enumerators of errc

[frederick-vs-ja]

No description provided.

[eisenwave]

I guess we could do the same in https://eel.is/c++draft/text.encoding.id

[frederick-vs-ja]

I guess we could do the same in https://eel.is/c++draft/text.encoding.id

Seems belonging to another PR to me. I'm working on this.

Thanks to using enum id;, these enumerators can be referred as both std::text_encoding::id::meow and std::text_encoding::meow. Do we want to index them as members of text_encoding::id or text_encoding?

[jwakely]

I think they should be indexed as members of text_encoding because that's the intended way to refer to them, but I don't have any preference whether they're also indexed under id.