fix -Wrange-loop-analysis in Apple clang version 12.0.0 (clang-1200.0.32.29) (#11240)

Summary:
Fix complain
```
db/db_impl/db_impl_compaction_flush.cc:417:19: error: loop variable 'bg_flush_arg' of type 'const rocksdb::DBImpl::BGFlushArg' creates a copy from type
      'const rocksdb::DBImpl::BGFlushArg' [-Werror,-Wrange-loop-analysis]
  for (const auto bg_flush_arg : bg_flush_args) {
                  ^
db/db_impl/db_impl_compaction_flush.cc:417:8: note: use reference type 'const rocksdb::DBImpl::BGFlushArg &' to prevent copying
  for (const auto bg_flush_arg : bg_flush_args) {
       ^~~~~~~~~~~~~~~~~~~~~~~~~
                  &
db/db_impl/db_impl_compaction_flush.cc:2911:21: error: loop variable 'bg_flush_arg' of type 'const rocksdb::DBImpl::BGFlushArg' creates a copy from type
      'const rocksdb::DBImpl::BGFlushArg' [-Werror,-Wrange-loop-analysis]
    for (const auto bg_flush_arg : bg_flush_args) {
                    ^
db/db_impl/db_impl_compaction_flush.cc:2911:10: note: use reference type 'const rocksdb::DBImpl::BGFlushArg &' to prevent copying
    for (const auto bg_flush_arg : bg_flush_args) {
         ^~~~~~~~~~~~~~~~~~~~~~~~~
                    &
```
from

```sh
xxx@MacBook-Pro / % g++ -v
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 12.0.0 (clang-1200.0.32.29)
Target: x86_64-apple-darwin21.6.0
Thread model: posix
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11240

Reviewed By: cbi42

Differential Revision: D43458729

Pulled By: ajkr

fbshipit-source-id: 26e110f83451509463a1bc308f737ccb693c9f45

db/db_impl/db_impl_compaction_flush.cc

@@ -414,7 +414,7 @@ Status DBImpl::AtomicFlushMemTablesToOutputFiles(
     assert(cfd->imm()->NumNotFlushed() != 0);
     assert(cfd->imm()->IsFlushPending());
   }
-  for (const auto bg_flush_arg : bg_flush_args) {
+  for (const auto& bg_flush_arg : bg_flush_args) {
     assert(bg_flush_arg.flush_reason_ == bg_flush_args[0].flush_reason_);
   }
 #endif /* !NDEBUG */
@@ -2908,7 +2908,7 @@ Status DBImpl::BackgroundFlush(bool* made_progress, JobContext* job_context,
 // All the CFD/bg_flush_arg in the FlushReq must have the same flush reason, so
 // just grab the first one
 #ifndef NDEBUG
-    for (const auto bg_flush_arg : bg_flush_args) {
+    for (const auto& bg_flush_arg : bg_flush_args) {
       assert(bg_flush_arg.flush_reason_ == bg_flush_args[0].flush_reason_);
     }
 #endif /* !NDEBUG */