Require a complete type when enumerating the fields of a class. (#2956)

Fixes a crash on invalid found by fuzzing.

explorer/testdata/class/fail_incomplete_in_conversion.carbon

@@ -0,0 +1,15 @@
+// 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
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+class C {
+  // CHECK:STDERR: COMPILATION ERROR: fail_incomplete_in_conversion.carbon:[[@LINE+2]]: type error in `as`: `{}` is not explicitly convertible to `class C`:
+  // CHECK:STDERR: incomplete type `class C` used in implicit conversion
+  var x: ({} as C);
+}
+
+fn Main() -> i32 { return 0; }