`LockIsolated.withValue` closure should be sendable

[stephencelis]

Brought up by @kabiroberai in Slack with this demo of the issue:

class Bad {
  var i = 0
  func racy() { i += 1 }
}

let bad = LockIsolated(Bad())

var _bad: Bad?
bad.withValue { _bad = $0 }
_bad?.racy()  // not lock-isolated!