Finish up some fixes related to <rdar://problem/6497631> Message lookup is...

Finish up some fixes related to <rdar://problem/6497631> Message lookup is sometimes different than gcc's.

- Disallow casting 'super'. GCC allows this, however it doesn't make sense (super isn't an expression and the cast won't alter lookup/dispatch).
- Tighten up lookup when messaging 'self'.

llvm-svn: 66033