In incompatible pointer-typed ?: expressions, add implicit conversion

of RHSs to id type instead of void* if either has Objective-C object
type.
 - This ensures the result can still be used in normal places an
   object can be used, like a message send.

Add implicit conversions for ?: applied to qualified id types to
ensure that the RHSs are compatible. 
 - This prevents a codegen crash (creating invalid PHI nodes).
 - Again, this relates to the fact that qualified id types have no
   canonical types.
 - Note that the implicit type casted to is incorrect, however this
   doesn't currently cause problems because of the flexibility of the
   id type.

Test cases for above.

llvm-svn: 55346