Allow reference binding of a reference of Objective-C object type to

Allow reference binding of a reference of Objective-C object type to
an lvalue of another, compatible Objective-C object type (e.g., a
subclass). Introduce a new initialization sequence step kind to
describe this binding, along with a new cast kind. Fixes PR7741.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@110513 91177308-0d34-0410-b5e6-96231b3b80d8