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/control_flow.cpp

@@ -157,7 +157,9 @@ static auto AddCleanups(Context& context, ScopeStack::CleanupScopeDepth depth)
     // cleanup blocks, so we'll want to avoid this in the future.
     BuildUnaryOperator(context,
                        context.insts().GetLocIdForDesugaring(destroy_id),
-                       {.interface_name = CoreIdentifier::Destroy}, destroy_id);
+                       {.interface_name = CoreIdentifier::Destroy,
+                        .op_name = CoreIdentifier::SelfDestruct},
+                       destroy_id);
   }
 }