Move std::atomic_flag to std::atomic<bool>

C++20 accidentally deprecated ATOMIC_FLAG_INIT, but in C++17 this is the
only way to reliably initialize an atomic_flag to a known value.
See https://en.cppreference.com/w/cpp/atomic/ATOMIC_FLAG_INIT

src/snmalloc/ds/aba.h

@@ -141,7 +141,7 @@ namespace snmalloc
   class ABA
   {
     std::atomic<T*> ptr = nullptr;
-    std::atomic_flag lock = ATOMIC_FLAG_INIT;
+    std::atomic<bool> lock{false};
 
   public:
     // This method is used in Verona
@@ -154,7 +154,7 @@ namespace snmalloc
 
     Cmp read()
     {
-      while (lock.test_and_set(std::memory_order_acquire))
+      while (lock.exchange(true, std::memory_order_acquire))
         Aal::pause();
 
 #  if !defined(NDEBUG) && !defined(SNMALLOC_DISABLE_ABA_VERIFY)
@@ -184,7 +184,7 @@ namespace snmalloc
 
       ~Cmp()
       {
-        parent->lock.clear(std::memory_order_release);
+        parent->lock.store(false, std::memory_order_release);
 #  if !defined(NDEBUG) && !defined(SNMALLOC_DISABLE_ABA_VERIFY)
         operation_in_flight = false;
 #  endif

src/snmalloc/mem/pooled.h

@@ -24,25 +24,25 @@ namespace snmalloc
     std::atomic<T*> next{nullptr};
     /// Used by the pool to keep the list of all entries ever created.
     capptr::Alloc<T> list_next;
-    std::atomic_flag in_use = ATOMIC_FLAG_INIT;
+    std::atomic<bool> in_use{false};
 
   public:
     void set_in_use()
     {
-      if (in_use.test_and_set())
+      if (in_use.exchange(true))
         error("Critical error: double use of Pooled Type!");
     }
 
     void reset_in_use()
     {
-      in_use.clear();
+      in_use.store(false);
     }
 
     bool debug_is_in_use()
     {
-      bool result = in_use.test_and_set();
+      bool result = in_use.exchange(true);
       if (!result)
-        in_use.clear();
+        in_use.store(false);
       return result;
     }
   };