swift234

[serialization] Add support for TupleTypes.

These are implementing using trailing TUPLE_TYPE_ELT records after the
initial TUPLE_TYPE record. This is the next step towards function decls.

Element initializers are silently ignored for now.

Also, do serialize a record for IdentifierTypes, even though we're not
including the components yet, so that we don't serialize the underlying
type more than once.

Swift SVN r5683