swift234

[serialization] Add support for slice types and variadic functions.

This is the first time reaching the case where the module refers to a type
that also appears in the TU and tries to modify it. This shouldn't be a
problem for any of the other types we currently serialize besides
BoundGenericType and ArraySliceType because we either don't try to modify
them after creation or are guaranteed a new type object.

Swift SVN r6060