[Type check diagnostics] Add already-fixed test case from rdar://problem/56710317.

Ensures that we don't regress diagnostics for this case in the future.

Author: DougGregor

validation-test/Sema/SwiftUI/rdar56710317.swift

@@ -0,0 +1,25 @@
+// RUN: %target-typecheck-verify-swift -target x86_64-apple-macosx10.15 -swift-version 5
+// REQUIRES: objc_interop
+
+import SwiftUI
+import Combine
+
+final class MyObservableObject: ObservableObject {
+    @Published private(set) var isDoingTheThing = false
+}
+
+struct MyView: View {
+    @EnvironmentObject var observableObject: MyObservableObject
+            
+    var body: some View {
+        MyBindingView(doTheThing: $observableObject.isDoingTheThing) // expected-error{{cannot assign to property: 'isDoingTheThing' setter is inaccessible}}
+    }
+}
+
+struct MyBindingView: View {
+    @Binding var doTheThing: Bool
+    
+    var body: some View {
+        Text(doTheThing ? "Doing The Thing" : "Doing Sweet Nothing")
+    }
+}