[clang][deps] Use lock_guard instead of unique_lock

[jansvoboda11]

Summary

This patch changes uses of std::unique_lock to std::lock_guard.

The std::unique_lock template provides some advanced capabilities (deferred locking, time-constrained locking attempts, etc.) we don't use in the caching filesystem. Plain std::lock_guard will do here.

[dexonsmith]

LGTM

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

DependencyScanningFilesystemSharedCache::SingleCache::SingleCache() {
   NumShards =
       std::max(2u, llvm::hardware_concurrency().compute_thread_count() / 4);
   CacheShards = std::make_unique<CacheShard[]>(NumShards);
 }
 
 DependencyScanningFilesystemSharedCache::SharedFileSystemEntry &
 DependencyScanningFilesystemSharedCache::SingleCache::get(StringRef Key) {
   CacheShard &Shard = CacheShards[llvm::hash_value(Key) % NumShards];
-  std::unique_lock<std::mutex> LockGuard(Shard.CacheLock);
+  std::lock_guard<std::mutex> LockGuard(Shard.CacheLock);
   auto It = Shard.Cache.try_emplace(Key);
   return It.first->getValue();
 }
 
 DependencyScanningFilesystemSharedCache::SharedFileSystemEntry &
 DependencyScanningFilesystemSharedCache::get(StringRef Key, bool Minimized) {
   SingleCache &Cache = Minimized ? CacheMinimized : CacheOriginal;
   return Cache.get(Key);
DependencyScanningWorkerFilesystem::getOrCreateFileSystemEntry(
 
   // FIXME: Handle PCM/PCH files.
   // FIXME: Handle module map files.
 
   DependencyScanningFilesystemSharedCache::SharedFileSystemEntry
       &SharedCacheEntry = SharedCache.get(Filename, ShouldMinimize);
   const CachedFileSystemEntry *Result;
   {
-    std::unique_lock<std::mutex> LockGuard(SharedCacheEntry.ValueLock);
+    std::lock_guard<std::mutex> LockGuard(SharedCacheEntry.ValueLock);
     CachedFileSystemEntry &CacheEntry = SharedCacheEntry.Value;
 
     if (!CacheEntry.isValid()) {
       auto MaybeStatus = getUnderlyingFS().status(Filename);
       if (!MaybeStatus && !shouldCacheStatFailures(Filename))
         // HACK: We need to always restat non source files if the stat fails.
         //   This is because Clang first looks up the module cache and module
         //   files before building them, and then looks for them again. If we