These will never appear in the source language, but can arise
after substitution when the original type is a tuple type with
a pack expansion type.
Two examples:
- original type: (Int, T...), substitution T := {}
- original type: (T...), substitution T := {Int}
We need to model these correctly to maintain invariants.
Callers that previously used to rely on TupleType::get()
returning a ParenType now explicitly check for the one-element
case instead.
