Rework Sema::CheckConditionalOperands(). No functionality change.

This was necessary to simplify some other changes I'm making (wrt ObjC type cleanups).

The idea is to separate the constraint checks for block pointers, ObjC pointers, and C pointers (the previous code combined them into one clause).

Note: This routine will be further simplified when I integrate the ObjC type cleanups (forthcoming).
llvm-svn: 74604