[TypeChecker] NFC: Add a test-case for mixed SwiftUI + multi-statement closures

This test-case used to crash with multi-statement closure inference
enabled in result builder contexts.

Author: xedin

validation-test/Sema/SwiftUI/mixed_swiftui_and_multistatement_closures.swift

@@ -0,0 +1,87 @@
+// RUN: %target-typecheck-verify-swift -target %target-cpu-apple-macosx10.15 -swift-version 5 -experimental-multi-statement-closures
+// REQUIRES: objc_interop
+// REQUIRES: OS=macosx
+
+import SwiftUI
+
+enum Status {
+  case complete
+  case waiting
+}
+
+struct Item : Hashable {
+  var question: String
+  var answer: Int
+}
+
+func transform(_ v: Int) -> String? { return String(v) }
+func transform(_ v: Double) -> String? { return String(v) }
+
+struct MyView : View {
+  var status: Status
+
+  var items: [Item] = []
+
+  var currItem: Item {
+    get { fatalError() }
+    nonmutating set { }
+  }
+
+  var body: some View {
+    ZStack {
+      ItemsView {
+        EmptyView()
+      } results: {
+        switch (status) {
+        case .complete:
+          ForEach(items, id: \.self) { item in
+            if let question = item.question,
+               let answer = item.answer {
+              ItemView {
+                currItem.question = question
+                currItem.answer = answer
+              } content: {
+                AnswerView(title: "",
+                           color: .red,
+                           answer: transform(answer) ?? "",
+                           selected: false)
+              }
+            }
+          }
+
+        default:
+          EmptyView()
+        }
+      }
+    }
+  }
+}
+
+struct AnswerView : View {
+  var title: String
+  var color: Color
+  var answer: String
+  var selected: Bool
+
+  var body: some View {
+    EmptyView()
+  }
+}
+
+struct ItemsView<Content: View,  Results: View>: View {
+  @ViewBuilder var content: Content
+  @ViewBuilder var results: Results
+
+  var body: some View {
+    EmptyView()
+  }
+}
+
+struct ItemView<Content: View> : View {
+  var fn: () -> Void
+  @ViewBuilder var content: Content
+
+  var body: some View {
+    EmptyView()
+  }
+}