GCAdapter::UseAdapter() reads s_is_adapter_wanted, which gets
initialized by config_guard.~ConfigChangeCallbackGuard(). So we must
wait until after destroying the config guard to know whether we have any
controllers set to GC Adapter.
SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.