swift234

Thread fix-its all the way through the diagnostics machinery.

Fix-its are now working!

Feedback on the API is welcome. I mostly took what was in Clang as a model,
so the usual way to use a FixIt is to pipe it into an active diagnostic:

  << Diagnostic::FixIt::makeInsertion(Tok.getLoc(), "&")
  << Diagnostic::FixIt::makeDeletion(E->getRange())
  << Diagnostic::FixIt::makeReplacement(E->getRange(), "This")

(Yes, of course you can specify the first two in terms of makeReplacement,
but that's not as convenient or as communicative.)

I plan to extend the expected-* notation to include a notation for fix-its
before converting any other diagnostics over, but this is a start.

Swift SVN r4751