Merge #54: Fix clang thread-safety-pointer warnings

7daf4ed Fix clang thread-safety-pointer warnings (Cory Fields)

Pull request description:

  This prevents a warning in an upcoming version of clang which will turn this on for us.

  Submitted upstream: google#1270

  Clang's `-Wthread-safety-pointer` warnings will become part of `-Wthread-safety` in some future release. Add missing annotations for functions that pass the address of guarded members. No functional change. Fixes the following warnings:

  ```bash
  util/cache.cc:220:17: warning: passing pointer to variable 'in_use_' requires holding mutex 'mutex_' [-Wthread-safety-pointer]
    220 |     LRU_Append(&in_use_, e);
        |                 ^
  util/cache.cc:235:17: warning: passing pointer to variable 'lru_' requires holding mutex 'mutex_' [-Wthread-safety-pointer]
    235 |     LRU_Append(&lru_, e);
  ```

ACKs for top commit:
  davidgumberg:
    Tested and review ACK 7daf4ed
  fanquake:
    ACK 7daf4ed

Tree-SHA512: 5804c6fa915006b6fa75c6e31424058ddc5c5932ba126160aaf518325f04d158aef3e44c250e6b2edbd91aaf80a73f0dd48b367c8305ecaec92a088525d620d8

Author: fanquake

util/cache.cc

@@ -171,8 +171,8 @@ class LRUCache {
  private:
   void LRU_Remove(LRUHandle* e);
   void LRU_Append(LRUHandle* list, LRUHandle* e);
-  void Ref(LRUHandle* e);
-  void Unref(LRUHandle* e);
+  void Ref(LRUHandle* e) EXCLUSIVE_LOCKS_REQUIRED(mutex_);
+  void Unref(LRUHandle* e) EXCLUSIVE_LOCKS_REQUIRED(mutex_);
   bool FinishErase(LRUHandle* e) EXCLUSIVE_LOCKS_REQUIRED(mutex_);
 
   // Initialized before use.