Automerge: [ADT] Use std::scoped_lock in ConcurrentHashtable (NFC) (#165100)

This patch uses std::scoped_lock to ensure the mutex is released via
RAII, improving robustness.

Author: kazutakahirata

llvm/include/llvm/ADT/ConcurrentHashtable.h

@@ -177,7 +177,7 @@ class ConcurrentHashTableByPtr {
 
 #if LLVM_ENABLE_THREADS
     // Lock bucket.
-    CurBucket.Guard.lock();
+    std::scoped_lock<std::mutex> Lock(CurBucket.Guard);
 #endif
 
     HashesPtr BucketHashes = CurBucket.Hashes;
@@ -195,11 +195,6 @@ class ConcurrentHashTableByPtr {
 
         CurBucket.NumberOfEntries++;
         RehashBucket(CurBucket);
-
-#if LLVM_ENABLE_THREADS
-        CurBucket.Guard.unlock();
-#endif
-
         return {NewData, true};
       }
 
@@ -208,10 +203,6 @@ class ConcurrentHashTableByPtr {
         KeyDataTy *EntryData = BucketEntries[CurEntryIdx];
         if (Info::isEqual(Info::getKey(*EntryData), NewValue)) {
           // Already existed entry matched with inserted data is found.
-#if LLVM_ENABLE_THREADS
-          CurBucket.Guard.unlock();
-#endif
-
           return {EntryData, false};
         }
       }