Fix a bug in Sema::CheckConditionalOperands(). When mixing pointers and null pointer constants, we need to promote the null pointer constant (which is an integer) to the pointer type. Test case is self explanatory.

This surfaced yesterday, when compiling test/Sema/cocoa.m on Leopard. Since this has nothing to do with ObjC, it's kind of bizarre this hasn't shown up before. I imagine Cocoa.h on Leopard may have changed recently? 

Thanks to Ted for localizing the bug and giving me a useful AST dump...

llvm-svn: 43114