Fix rdar://6821047 - clang crashes on subscript of interface in 64-bit mode

Several changes here:
1. We change Type::isIncompleteType to realize that forward declared 
   interfaces are incomplete.  This eliminate special case code for this
   from the sizeof path, and starts us rejecting P[4] when P is a pointer
   to an incomplete interface.
2. Explicitly reject P[4] when P points to an interface in non-fragile ABI
   mode.
3. Switch the sizeof(interface) diagnostic back to an error instead of a 
   warning in non-fragile abi mode.

llvm-svn: 69943