TableGen: Let expressions available to list subscriptions and list slices

This enables indexing in `!foreach` and permutation with `list[permlist]`.

Enhancements in syntax:

  - `list<int>` is applicable as a slice element.
  - `list[int,]` is evaluated as not `ElemType` but `list<ElemType>`
    with a single element.

Part of D145872

FIXME: I didn't apply new semantics to BitSlice.