Recover better from invalid C++ classes. (#5992)

When importing a class definition, don't ask for the class layout if the
definition is invalid. Avoids an assertion failure in Clang.

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>

toolchain/check/testdata/interop/cpp/enum/invalid.carbon

@@ -0,0 +1,27 @@
+// 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
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/primitives.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/enum/invalid.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/enum/invalid.carbon
+
+// --- fail_invalid_underlying_type.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp inline '''
+// CHECK:STDERR: fail_invalid_underlying_type.carbon:[[@LINE+4]]:20: error: unknown type name 'no_such_type' [CppInteropParseError]
+// CHECK:STDERR:     9 | enum InvalidEnum : no_such_type {};
+// CHECK:STDERR:       |                    ^
+// CHECK:STDERR:
+enum InvalidEnum : no_such_type {};
+''';
+
+// Use of an invalid enum should not cause the toolchain to crash or produce
+// redundant diagnostics.
+var x: Cpp.InvalidEnum;