swift234

Add some logic to the type checking process to prevent stack overflows while type checking certain kinds of self-referential declarations.  This addresses rdar://problem/1641865, rdar://problem/16994520 and host of other radars that have been filed over the past several months.

Post-WWDC, we need to update the type checking process to make these kinds of infinite loops impossible without checking special flags.

Swift SVN r18598