[ADT] Use structured bindings (NFC) (#161627)

Both Size and Count are just integers, so I am not using & here.

Author: kazutakahirata

llvm/include/llvm/ADT/ConcurrentHashtable.h

@@ -253,9 +253,8 @@ class ConcurrentHashTableByPtr {
 
     OS << "\nOverall number of entries = " << OverallNumberOfEntries;
     OS << "\nOverall number of non empty buckets = " << NumberOfNonEmptyBuckets;
-    for (auto &BucketSize : BucketSizesMap)
-      OS << "\n Number of buckets with size " << BucketSize.first << ": "
-         << BucketSize.second;
+    for (auto [Size, Count] : BucketSizesMap)
+      OS << "\n Number of buckets with size " << Size << ": " << Count;
 
     std::stringstream stream;
     stream << std::fixed << std::setprecision(2)