Runtime: rewrite resource releasing in TLS dtor (#611)
Runtime: rewrite resource releasing in TLS dtor (#611)

`pthread_key_create` takes a destructor which is supposed to release
the resources associated with the key.  The code was doing that by
returning a consumed object.  Explicitly spell out the release instead.
tensorflow/swift-apis,tensorflow/swift-apis,tensorflow/swift-apis,tensorflow/swift-apis,tensorflow/swift-apis