Fix compile errors in Clang due to unused variables depending on the build configuration (#11234)

Summary:
This PR fixes compilation errors in Clang due to unused variables like the below:
```
[109/329] Building CXX object CMakeFiles/rocksdb.dir/db/version_edit_handler.cc.o
FAILED: CMakeFiles/rocksdb.dir/db/version_edit_handler.cc.o
ccache /opt/homebrew/opt/llvm/bin/clang++ -DGFLAGS=1 -DGFLAGS_IS_A_DLL=0 -DHAVE_FULLFSYNC -DJEMALLOC_NO_DEMANGLE -DLZ4 -DOS_MACOSX -DROCKSDB_JEMALLOC -DROCKSDB_LIB_IO_POSIX -DROCKSDB_NO_DYNAMIC_EXTENSION -DROCKSDB_PLATFORM_POSIX -DSNAPPY -DTBB -DZLIB -DZSTD -I/Users/jaepil/work/deepsearch/deps/cpp/rocksdb -I/Users/jaepil/work/deepsearch/deps/cpp/rocksdb/include -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -I/Users/jaepil/app/include -I/opt/homebrew/include -I/opt/homebrew/opt/llvm/include -I/opt/homebrew/opt/llvm/include/c++/v1 -W -Wextra -Wall -pthread -Wsign-compare -Wshadow -Wno-unused-parameter -Wno-unused-variable -Woverloaded-virtual -Wnon-virtual-dtor -Wno-missing-field-initializers -Wno-strict-aliasing -Wno-invalid-offsetof -fno-omit-frame-pointer -momit-leaf-frame-pointer -march=armv8-a+crc+crypto -Wno-unused-function -Werror -O2 -g -DNDEBUG -arch arm64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.1.sdk -std=gnu++20 -MD -MT CMakeFiles/rocksdb.dir/db/version_edit_handler.cc.o -MF CMakeFiles/rocksdb.dir/db/version_edit_handler.cc.o.d -o CMakeFiles/rocksdb.dir/db/version_edit_handler.cc.o -c /Users/jaepil/work/deepsearch/deps/cpp/rocksdb/db/version_edit_handler.cc
/Users/jaepil/work/deepsearch/deps/cpp/rocksdb/db/version_edit_handler.cc:30:10: error: variable 'recovered_edits' set but not used [-Werror,-Wunused-but-set-variable]
  size_t recovered_edits = 0;
         ^
1 error generated.
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11234

Reviewed By: cbi42

Differential Revision: D43458604

Pulled By: ajkr

fbshipit-source-id: d8c50e1a108887b037a120cd9f19374ddaeee817

cache/lru_cache.cc

@@ -103,7 +103,7 @@ void LRUHandleTable::Resize() {
   std::unique_ptr<LRUHandle* []> new_list {
     new LRUHandle* [size_t{1} << new_length_bits] {}
   };
-  uint32_t count = 0;
+  [[maybe_unused]] uint32_t count = 0;
   for (uint32_t i = 0; i < old_length; i++) {
     LRUHandle* h = list_[i];
     while (h != nullptr) {

db/internal_stats.cc

@@ -658,11 +658,6 @@ void InternalStats::CollectCacheEntryStats(bool foreground) {
                                              min_interval_factor);
 }
 
-std::function<void()> Blah() {
-  static int x = 42;
-  return [&]() { ++x; };
-}
-
 std::function<void(const Slice& key, Cache::ObjectPtr value, size_t charge,
                    const Cache::CacheItemHelper* helper)>
 InternalStats::CacheEntryRoleStats::GetEntryCallback() {

db/version_edit_handler.cc

@@ -27,7 +27,7 @@ void VersionEditHandlerBase::Iterate(log::Reader& reader,
   assert(log_read_status);
   assert(log_read_status->ok());
 
-  size_t recovered_edits = 0;
+  [[maybe_unused]] size_t recovered_edits = 0;
   Status s = Initialize();
   while (reader.LastRecordEnd() < max_manifest_read_size_ && s.ok() &&
          reader.ReadRecord(&record, &scratch) && log_read_status->ok()) {