For CF functions with 'Get' semantics, the compiler currently assumes that
the result is autoreleased and must be retained. It does so on all platforms
by emitting a call to objc_retainAutoreleasedReturnValue. On Darwin, this
is implemented by the ObjC runtime. On Linux, there is no runtime, and
therefore we have to stub it out here ourselves. The compiler will eventually
call swift_release to balance the retain below.
This is a workaround until the compiler no longer emits this callout on Linux.