Refine bind tuple pattern typing for named tuples

[noti0na1]

ef getNamedTuple: (x: Int, y: String) = (x = 42, y = "Hello")

getNamedTuple match
  case t @ (a, b) => 
    // Although the pattern is a regular tuple, we are not able to call `._i` on t
    t._1 // error, _1 is not a member of named tuple

[noti0na1]

If named tuple is a Product (#22037), then we may not need this fix. However, it is probably more difficult to make it a Product now.

[natsukagami]

It's a bit strange that capturing the pattern could change the type of the scrutinee into something incompatible 😓 but it seems unavoidable, LGTM