swift234

SILGen: Make initializing ctors return this back.
John recommended this because it has a number of benefits:
- It avoids a retain/release cycle, because 'this' can be passed to the initializer at +1 and just received back at +1.
- The allocating constructor can thus tail call the initializing constructor.
- If we end up needing to support rogue ObjC initializers that change self, we're set up to do so.

Swift SVN r3836