Add support for struct types, following p0561. (#856)

executable_semantics/testdata/struct/empty.carbon

@@ -0,0 +1,25 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: result: 0
+
+package ExecutableSemanticsTest api;
+
+fn main() -> i32 {
+  var empty: {} = {};
+  empty = {};
+  if (not (empty == {})) {
+    return 1;
+  }
+  match (empty) {
+    case {} => {
+      return 0;
+    }
+  }
+}