Fixed Sema::CheckEqualityOperands() and Sema::CheckRelationalOperands() to deal more 
thoughtfully with incompatible pointers. This includes:

- Emit a diagnostic when two pointers aren't compatible!
- Promote one of the pointers/integers so we maintain the invariant expected by the 
code generator (i.e. that the left/right types match).
- Upgrade the pointer/integer comparison diagnostic to include the types. 

llvm-svn: 41127