Add use-after-destruction error messages

Summary: `ConcurrentHashMap` has a `DCHECK` for use-after-destruction. One of these has a comment explaining what it's for, but `DCHECK` supports taking a message to show the user. Make it a message.

Reviewed By: yfeldblum, magedm

Differential Revision: D28609176

fbshipit-source-id: ea6e57b7650ad597f45ea3efc2fa0fb62d5de40e

folly/concurrency/detail/ConcurrentHashMap-detail.h

@@ -610,7 +610,7 @@ class alignas(64) BucketTable {
         break;
       }
     }
-    DCHECK(buckets);
+    DCHECK(buckets) << "Use-after-destruction by user.";
   }
 
   template <typename MatchFunc, typename K, typename... Args>
@@ -638,7 +638,7 @@ class alignas(64) BucketTable {
       bcount = bucket_count_.load(std::memory_order_relaxed);
     }
 
-    DCHECK(buckets); // Use-after-destruction by user.
+    DCHECK(buckets) << "Use-after-destruction by user.";
     auto idx = getIdx(bcount, h);
     auto head = &buckets->buckets_[idx]();
     auto node = head->load(std::memory_order_relaxed);