src: remove usage of std::shared_ptr<T>::unique()

`std::shared_ptr<T>::unique()` has been removed in C++20, so this change
uses `std::shared_ptr<T>::use_count()` instead which is available in
C++20.

Fixes: #47311
Signed-off-by: Darshan Sen <raisinten@gmail.com>
PR-URL: #47315
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node_threadsafe_cow-inl.h

@@ -0,0 +1,54 @@
+#ifndef SRC_NODE_THREADSAFE_COW_INL_H_
+#define SRC_NODE_THREADSAFE_COW_INL_H_
+
+#if defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
+namespace node {
+
+template <typename T>
+T* CopyOnWrite<T>::write() {
+  if (data_.use_count() > 1l) {
+    data_ = std::make_shared<T>(*data_);
+  }
+  return data_.get();
+}
+
+template <typename T>
+ThreadsafeCopyOnWrite<T>::Read::Read(const ThreadsafeCopyOnWrite<T>* cow)
+    : cow_(cow), lock_(cow->impl_->mutex) {}
+
+template <typename T>
+const T& ThreadsafeCopyOnWrite<T>::Read::operator*() const {
+  return cow_->impl_->data;
+}
+
+template <typename T>
+const T* ThreadsafeCopyOnWrite<T>::Read::operator->() const {
+  return &cow_->impl_->data;
+}
+
+template <typename T>
+ThreadsafeCopyOnWrite<T>::Write::Write(ThreadsafeCopyOnWrite<T>* cow)
+    : cow_(cow), impl_(cow->impl_.write()), lock_(impl_->mutex) {}
+
+template <typename T>
+T& ThreadsafeCopyOnWrite<T>::Write::operator*() {
+  return impl_->data;
+}
+
+template <typename T>
+T* ThreadsafeCopyOnWrite<T>::Write::operator->() {
+  return &impl_->data;
+}
+
+template <typename T>
+ThreadsafeCopyOnWrite<T>::Impl::Impl(const Impl& other) {
+  RwLock::ScopedReadLock lock(other.mutex);
+  data = other.data;
+}
+
+}  // namespace node
+
+#endif  // defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
+#endif  // SRC_NODE_THREADSAFE_COW_INL_H_