Fix compilation errors, clang++-15 (#10907)

Summary:
I've tried to compile the main branch, but there are two minor things which are make CE.
I'm not sure about the second one (`num_empty_non_l0_level`), probably there is should be additional assert.

```
-c ../cache/clock_cache.cc
[build] ../cache/clock_cache.cc:855:15: error: variable 'i' set but not used [-Werror,-Wunused-but-set-variable]
[build]   for (size_t i = 0; &array_[current] != h; i++) {
[build]               ^
```

```
[build] ../db/version_set.cc:3665:7: error: variable 'num_empty_non_l0_level' set but not used [-Werror,-Wunused-but-set-variable]
[build]   int num_empty_non_l0_level = 0;
[build]       ^
[build] 1 error generated.
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10907

Reviewed By: jay-zhuang

Differential Revision: D40866667

Pulled By: ajkr

fbshipit-source-id: 963b7bd56859d0b3b2779cd36fad229425cb7b17

cache/clock_cache.cc

@@ -852,7 +852,7 @@ void ClockHandleTable::Rollback(const UniqueId64x2& hashed_key,
                                 const ClockHandle* h) {
   size_t current = ModTableSize(hashed_key[1]);
   size_t increment = static_cast<size_t>(hashed_key[0]) | 1U;
-  for (size_t i = 0; &array_[current] != h; i++) {
+  while (&array_[current] != h) {
     array_[current].displacements.fetch_sub(1, std::memory_order_relaxed);
     current = ModTableSize(current + increment);
   }

db/version_set.cc

@@ -3662,13 +3662,9 @@ void VersionStorageInfo::SetFinalized() {
     assert(MaxBytesForLevel(level) >= max_bytes_prev_level);
     max_bytes_prev_level = MaxBytesForLevel(level);
   }
-  int num_empty_non_l0_level = 0;
   for (int level = 0; level < num_levels(); level++) {
     assert(LevelFiles(level).size() == 0 ||
            LevelFiles(level).size() == LevelFilesBrief(level).num_files);
-    if (level > 0 && NumLevelBytes(level) > 0) {
-      num_empty_non_l0_level++;
-    }
     if (LevelFiles(level).size() > 0) {
       assert(level < num_non_empty_levels());
     }