Merge pull request #35383 from mikeash/fix-concurrentarray-memory-order3

[Runtime] Make Concurrent.h ReaderCount loads seq_cst as well.

Author: mikeash

include/swift/Runtime/Concurrent.h

@@ -538,7 +538,7 @@ template <class ElemTy> struct ConcurrentReadableArray {
     new(&storage->data()[count]) ElemTy(elem);
     storage->Count.store(count + 1, std::memory_order_release);
 
-    if (ReaderCount.load(std::memory_order_acquire) == 0)
+    if (ReaderCount.load(std::memory_order_seq_cst) == 0)
       deallocateFreeList();
   }
 
@@ -853,7 +853,7 @@ struct ConcurrentReadableHashMap {
   /// Free all the arrays in the free lists if there are no active readers. If
   /// there are active readers, do nothing.
   void deallocateFreeListIfSafe() {
-    if (ReaderCount.load(std::memory_order_acquire) == 0)
+    if (ReaderCount.load(std::memory_order_seq_cst) == 0)
       FreeListNode::freeAll(&FreeList);
   }