Add helper to get flattened tuple types

The API on TupleType::getFlattenedTypes follows our normal conventions by accepting an output parameter, but requires callers to allocate their own storage and lends itself to use in an imperative style. This makes it difficult to use in tablegen. The current solution is to define a lambda that is immediately called, but it's cleaner to extract that into a helper.

PiperOrigin-RevId: 258672046