Implement implicit conversions for Objective-C specific types, e.g.,

converting a pointer to one Objective-C interface into a pointer to another
Objective-C interface, and conversions with 'id'. The semantics seems
to match GCC, although they seem somewhat ad hoc.

Fixed a few cases where we assumed the C++ definition of isObjectType,
but were getting the C definition, causing failures in trouble with
conversions to void pointers.

llvm-svn: 60130