swift234

IRGen: Clean up class archetype spare bits hack

We were calling hasTypeParameter() on the interface type of the enum
element. Since enum elements are case constructors now, the interface
type was a GenericFunctionType, and since conceptually these cannot
contain free type variables, this would always return to false.

The right fix here is to pass down the unsubstituted type info and
look at the spare bits of that when doing multi-payload enum layout.

Now that this works, we can remove a FIXME that was added to patch
around this.