Replace std::lock_guard and std::lock with std::scoped_lock (#1758)

std::scoped_lock was introduced in C++17 and is strictly better than
std::lock_guard as it supports locking any number of mutexes safely.
It's also easier to use than std::lock for locking multiple mutexes at
once.

wpiutil/src/main/native/cpp/SafeThread.cpp

@@ -29,7 +29,7 @@ detail::SafeThreadOwnerBase::~SafeThreadOwnerBase() {
 }
 
 void detail::SafeThreadOwnerBase::Start(std::shared_ptr<SafeThread> thr) {
-  std::lock_guard lock(m_mutex);
+  std::scoped_lock lock(m_mutex);
   if (auto thr = m_thread.lock()) return;
   m_stdThread = std::thread([=] { thr->Main(); });
   thr->m_threadId = m_stdThread.get_id();
@@ -37,7 +37,7 @@ void detail::SafeThreadOwnerBase::Start(std::shared_ptr<SafeThread> thr) {
 }
 
 void detail::SafeThreadOwnerBase::Stop() {
-  std::lock_guard lock(m_mutex);
+  std::scoped_lock lock(m_mutex);
   if (auto thr = m_thread.lock()) {
     thr->m_active = false;
     thr->m_cond.notify_all();
@@ -63,26 +63,24 @@ void detail::SafeThreadOwnerBase::Join() {
 void detail::swap(SafeThreadOwnerBase& lhs, SafeThreadOwnerBase& rhs) noexcept {
   using std::swap;
   if (&lhs == &rhs) return;
-  std::lock(lhs.m_mutex, rhs.m_mutex);
-  std::lock_guard lock_lhs(lhs.m_mutex, std::adopt_lock);
-  std::lock_guard lock_rhs(rhs.m_mutex, std::adopt_lock);
+  std::scoped_lock lock(lhs.m_mutex, rhs.m_mutex);
   std::swap(lhs.m_stdThread, rhs.m_stdThread);
   std::swap(lhs.m_thread, rhs.m_thread);
 }
 
 detail::SafeThreadOwnerBase::operator bool() const {
-  std::lock_guard lock(m_mutex);
+  std::scoped_lock lock(m_mutex);
   return !m_thread.expired();
 }
 
 std::thread::native_handle_type
 detail::SafeThreadOwnerBase::GetNativeThreadHandle() {
-  std::lock_guard lock(m_mutex);
+  std::scoped_lock lock(m_mutex);
   return m_stdThread.native_handle();
 }
 
 std::shared_ptr<SafeThread> detail::SafeThreadOwnerBase::GetThreadSharedPtr()
     const {
-  std::lock_guard lock(m_mutex);
+  std::scoped_lock lock(m_mutex);
   return m_thread.lock();
 }