For an index expression list, ListExpr::InitVal() passed the TypeList
to Expr::InitVal() for each expression element in the list instead of
the type for that element. This made RecordConstructorExpr::InitVal()
complain since it expects a RecordType and not a TypeList with a
RecordType element as an argument. In most other cases, Expr::InitVal()
worked because check_and_promote() "flattens" the list to a single type.