Avoid crashing in custom witness for FacetTypes and symbolic object representations (#7033)

The type must be complete to look for a witness for Destroy. Do this
check through type completion rather than just checking to see if the
ClassInfo says the definition is closed, since completing the type has
side effects (resolves the self specific definition).

Then look for whether the class is abstract through the CompleteTypeInfo
instead of just looking at the inheritance type on ClassInfo, like type
completion does.

Last, FacetTypes are trivially destroyed just like TypeType.

toolchain/check/testdata/impl/custom_witness/destroy.carbon

@@ -0,0 +1,79 @@
+// 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/int.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/impl/custom_witness/destroy.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/impl/custom_witness/destroy.carbon
+
+// --- destroy_type.carbon
+library "[[@TEST_NAME]]";
+
+fn F() {
+  // This looks for a Destroy witness for a TypeType.
+  type as Core.Destroy;
+}
+
+// --- destroy_facet_type.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+
+fn F() {
+  // This looks for a Destroy witness for a FacetType.
+  (Z & Core.Destroy) as Core.Destroy;
+}
+
+// --- destroy_int_literal_type.carbon
+library "[[@TEST_NAME]]";
+
+fn F() {
+  // This looks for a Destroy witness for an IntLiteralType.
+  Core.IntLiteral() as Core.Destroy;
+}
+
+// --- destroy_int_type.carbon
+library "[[@TEST_NAME]]";
+
+fn F() {
+  // This looks for a Destroy witness for a class type that adapts IntType, and
+  // has a symbolic CompleteTypeWitness.
+  Core.Int(32) as Core.Destroy;
+}
+
+// --- destroy_symbolic_int_type.carbon
+library "[[@TEST_NAME]]";
+
+fn F(N:! Core.IntLiteral()) {
+  // This looks for a Destroy witness for a class type that adapts IntType with a
+  // symbolic type.
+  Core.Int(N) as Core.Destroy;
+}
+
+// --- destroy_class_type.carbon
+library "[[@TEST_NAME]]";
+
+class C(T:! type) {}
+
+fn F() {
+  // This looks for a Destroy witness for a class type.
+  C({}) as Core.Destroy;
+}
+
+// --- fail_destroy_incomplete_class.carbon
+library "[[@TEST_NAME]]";
+
+class C(T:! type);
+
+fn F() {
+  // This looks for a Destroy witness for an incomplete class type.
+  // CHECK:STDERR: fail_destroy_incomplete_class.carbon:[[@LINE+4]]:3: error: cannot convert type `C({})` into type implementing `Core.Destroy` [ConversionFailureTypeToFacet]
+  // CHECK:STDERR:   C({}) as Core.Destroy;
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  C({}) as Core.Destroy;
+}