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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@60130 91177308-0d34-0410-b5e6-96231b3b80d8