[tuple], [variant] Use pack indexing

[eisenwave]

https://eel.is/c++draft/tuple.cnstr and https://eel.is/c++draft/variant.ctor#1 provide an invented syntax $T_i$ to denote the $i^{th}$ type in a pack Types.

This invented syntax is completely unnecessary now that we have pack indexing; we could simply write Types...[i], and probably should do so.

[frederick-vs-ja]

In addition to Types...[i], we can also use fold expressions at some places.

[eisenwave]

Marking this decision-required because while it seems like a nice change, it's pretty debatable whether we want such a modernization. The old wording isn't broken, just not spelled how we would do it nowadays.

[jwakely]

In addition to Types...[i], we can also use fold expressions at some places.

I've specifically avoided doing that in some recent lwg issue wording, to be consistent with the existing text.

I would like to see this modernized, but not when we're under so much time pressure for C++26.