Add Destroy.SubobjectDestroy as a temporary replacement for Destroy.Op (#7773)

This change partially implements [PR #7362], which revises how objects
are destroyed. It is a partial implementation for two reasons:

1. This change moves `Destroy.Op`'s current behaviour into
`Destroy.SubobjectDestroy`, but it doesn't add support for objects with
non-trivial destruction.
2. `Destroy.SubobjectDestroy` is a workaround for `require impls
SubobjectDestroy`. We aren't able to use the latter until the dependents
add their requirements' implementations to their own witness tables.

[PR #7362]: https://github.com/carbon-language/carbon-lang/pulls/7362

toolchain/check/cpp/impl_lookup.cpp

@@ -241,7 +241,7 @@ static auto BuildDefaultWitness(
                             query_specific_interface, {fn_id});
 }
 
-static auto BuildDestroyWitness(
+static auto BuildCppDestroyWitness(
     Context& context, SemIR::LocId loc_id,
     SemIR::ConstantId query_self_const_id,
     SemIR::SpecificInterface query_specific_interface) -> SemIR::InstId {
@@ -264,8 +264,11 @@ static auto BuildDestroyWitness(
   if (fn_id == SemIR::ErrorInst::InstId || fn_id == SemIR::InstId::None) {
     return fn_id;
   }
-  return BuildCustomWitness(context, loc_id, query_self_const_id,
-                            query_specific_interface, {fn_id});
+  return BuildDestroyWitness(
+      context, loc_id,
+      GetFacetAccessType(
+          context, context.constant_values().GetInstId(query_self_const_id)),
+      query_self_const_id, query_specific_interface, {fn_id});
 }
 
 // Attempts to build a witness table entry for a C++ unary operator.
@@ -610,8 +613,8 @@ auto LookupCppImpl(Context& context, SemIR::LocId loc_id,
       return BuildDefaultWitness(context, loc_id, query_self_const_id,
                                  query_specific_interface);
     case SemIR::CoreInterface::Destroy:
-      return BuildDestroyWitness(context, loc_id, query_self_const_id,
-                                 query_specific_interface);
+      return BuildCppDestroyWitness(context, loc_id, query_self_const_id,
+                                    query_specific_interface);
 
     case SemIR::CoreInterface::CppRangeForIterate:
       return BuildCppRangeForIterateWitness(