Produce a class value when converting {} to class type. (#1265)

Previously we missed this special case because {} results in a
StructType value not a StructValue value.

explorer/testdata/class/empty_class.carbon

@@ -0,0 +1,21 @@
+// 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: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 0
+
+package ExplorerTest api;
+
+class A { fn F[me: Self]() -> i32 { return 0; } }
+
+fn Main() -> i32 {
+  // {} is a bit strange; its type is {}. Make sure we can properly convert a
+  // value of type {} to type TypeOfClassType.
+  var a: A = {};
+  return a.F();
+}