s/const auto/const auto&/ when doing loop (#6477)

Summary:
this silences following warning from clang-11
```
rocksdb/db/db_impl/db_impl_compaction_flush.cc:1040:21: warning: loop variable 'newf' of type 'const std::pair<int, rocksdb::FileMetaData>' creates a copy from type 'const
std::pair<int\
, rocksdb::FileMetaData>' [-Wrange-loop-analysis]
    for (const auto newf : c->edit()->GetNewFiles()) {
                    ^
rocksdb/db/db_impl/db_impl_compaction_flush.cc:1040:10: note: use reference type 'const std::pair<int, rocksdb::FileMetaData> &' to prevent copying
    for (const auto newf : c->edit()->GetNewFiles()) {
         ^~~~~~~~~~~~~~~~~
                    &
```
Signed-off-by: Kefu Chai <tchaikov@gmail.com>
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6477

Differential Revision: D20211850

Pulled By: ltamasi

fbshipit-source-id: 3e89e13a12bba79f1b934d46b7c4c0576cdafb01

db/db_impl/db_impl_compaction_flush.cc

@@ -1037,7 +1037,7 @@ Status DBImpl::CompactFilesImpl(
   }
 
   if (output_file_names != nullptr) {
-    for (const auto newf : c->edit()->GetNewFiles()) {
+    for (const auto& newf : c->edit()->GetNewFiles()) {
       (*output_file_names)
           .push_back(TableFileName(c->immutable_cf_options()->cf_paths,
                                    newf.second.fd.GetNumber(),
@@ -1137,7 +1137,7 @@ void DBImpl::NotifyOnCompactionBegin(ColumnFamilyData* cfd, Compaction* c,
         }
       }
     }
-    for (const auto newf : c->edit()->GetNewFiles()) {
+    for (const auto& newf : c->edit()->GetNewFiles()) {
       const FileMetaData& meta = newf.second;
       const FileDescriptor& desc = meta.fd;
       const uint64_t file_number = desc.GetNumber();