swift234

[Driver] Provide a single API to iterate over all valid file types.

Previously we had three separate instances of iterating from TY_INVALID+1
to TY_LAST, completely breaking type safety. Now we have a nice little
wrapper that takes a closure, which should inline down to the same thing
anyway.

Also, eliminate TY_LAST and just use TY_INVALID as our sentinel.

Swift SVN r23222