Constant CFString merge

Okay, sorry it took me a while to get to this. I pushed the parts of the
patch which rip out the constant CFString in CGObjCMac.cpp. I didn't
add the 64-bit struct change because this is just going to assert in
codegen. The 64-bit changes should go in as a consistent patch I
think.

- Daniel

Thank you.

I will have a look at the 64 bit change, and I will try to do a consistent patch.