title: Clearing SwiftUI TextField will not restore placeholder
body: <p>I have a SwiftUI screen with three textfields. When you run the code and tap the Clear button, you'll see three completely empty textfields. Expected is that you'd see the placeholder text, but that only appears in each textfield when it receives focus (i.e. user taps inside the field).</p>
  
  <pre><code>class UserInput: ObservableObject {
      @Published var text1 = "some text"
      @Published var text2 = "some more text"
      @Published var text3 = "and this is the final input"
  
      func clear() {
          self.text1 = ""
          self.text2 = ""
          self.text3 = ""
      }
  }
  
  struct ContentView: View {
      @ObservedObject var userInput = UserInput()
  
      var body: some View {
          Form {
              TextField("Type something in text1", text: self.$userInput.text1)
              TextField("Type something in text2", text: self.$userInput.text2)
              TextField("Type something in text3", text: self.$userInput.text3)
              Button("Clear all fields", action: self.userInput.clear)
          }
      }
  }
  </code></pre>
  
  <p>Is there something I'm missing, or is there a workaround for this behavior?</p>