[clang] [Clang][C++26] Implement "Ordering of constraints involving fold expressions (PR #98160)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Changes LGTM but this is complicated enough that it would be nice if @hubert-reinterpretcast could also take a look, because @erichkeane isn't back from leave yet.

https://github.com/llvm/llvm-project/pull/98160