pr_objc: Expose objc_retainAutoreleasedReturnValue()

- This allows it to be used by Foundation so that the duplicate
  function can be removed.

Author: spevans

src/swift/DispatchStubs.cc

@@ -199,8 +199,17 @@ SOURCE(VNODE)
 #endif
 SOURCE(WRITE)
 
-// See comment in CFFuntime.c explaining why objc_retainAutoreleasedReturnValue is needed.
+
+// 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.
 extern "C" void swift_retain(void *);
+
+SWIFT_CC(swift) DISPATCH_RUNTIME_STDLIB_INTERFACE
 extern "C" void * objc_retainAutoreleasedReturnValue(void *obj) {
     if (obj) {
         swift_retain(obj);