CodeGen: annotate ObjC ARC functions with ABI constraints

Certain ARC runtime functions have an ABI contract of being forwarding.
Annotate the functions with the appropriate `returned` attribute on the
arguments.  This hoists some of the runtime ABI contract information
into the frontend rather than the backend transformations.

The test adjustments are to mark the returned function parameter as
such.  The minor change to the IR output is due to the fact that the
returned reference of the object causes it to extend the lifetime of the
object by returning an autoreleased return value.  The result is that
the explicit objc_autorelease call is no longer formed, as autorelease
elision is now possible on the return.

llvm-svn: 294872