swift234

Change NULL/Nil/nil and various other macros to be imported as an unavailable decl,
instead of importing them as _Nil (which will be going away when nil becomes an expr).

You now get an error saying:
error: 'NULL' is unavailable: use 'nil' instead of this imported macro

This is pretty cool all around, except for the GCD macros that need to be fixed, I'll
handle that as a follow-up.


Swift SVN r18747