Intrinsic support for pinning.

Using the intrinsics is obnoxious because I needed them
to return Builtin.NativeObject?, but there's no reasonable
way to safely generate optional types from Builtins.cpp.
Ugh.

Dave and I also decided that there's no need for
swift_tryPin to allow a null object.

Swift SVN r23824

stdlib/runtime/HeapObject.cpp

@@ -303,7 +303,7 @@ void swift::swift_weakRelease(HeapObject *object) {
 }
 
 HeapObject *swift::swift_tryPin(HeapObject *object) {
-  if (!object) return nullptr;
+  assert(object);
 
   // Try to set the flag.  If this succeeds, the caller will be
   // responsible for clearing it.