swift234

Eliminate default argument expressions from the type system.

Elements of a tuple type now know if there is a default argument, and
what kind of default argument it is (callee side, __FILE__, __LINE__,
__COLUMN__), but they don't have an actual expression. There are a
number of cleanups this enables that will follow. 

Note that the serialization support is as-yet-untested.


Swift SVN r6351