swift234

rework lowering of 'self' in class init methods to use normal cleanups and variable
emission instead of hand coded magic.  Eliminating special cases allows simplifying
other parts of the compiler, and this probably helps the error handling initiative 
as well.

This uses the (horrible) new null_class instruction to properly model rebinding of
self.  The code that SILGen was producing before was wildly incorrect and we only
got lucky that it seemed to work in most cases before.

NFC except that SILGen tests see the new null_class instructions, and we get better
location info for 'return nil' for obscure reasons that don't really matter.



Swift SVN r27530