Consts in const generics should be pretty-printed

[varkor]

Often consts are debug-printed (e.g. ConstValue::Unevaluated(..)), rather than being pretty-printed. I don't have an exhaustive list of places that need to be fixed here, but I've noticed it in some errors.

[varkor]

These issues may be partially addressed by #59276.

[varkor]

I'm going to close this, because I think #59276 has addressed most of the issues. If we encounter any more, it should be addressed by replacing the debug printing with normal printing of consts.