Increase accuracy of __builtin_object_size.

Improvements:

- For all types, we would give up in a case such as:
    __builtin_object_size((char*)&foo, N);
  even if we could provide an answer to
    __builtin_object_size(&foo, N);
  We now provide the same answer for both of the above examples in all
  cases.

- For type=1|3, we now support subobjects with unknown bases, as long
  as the designator is valid.

Thanks to Richard Smith for the review + design planning.

Review: http://reviews.llvm.org/D12169
llvm-svn: 246877