Enable -Wunused-variable in glean/PACKAGE

Summary:
This diff enables compilation warning flags for the directory in question. Further details are in [this workplace post](https://fb.workplace.com/permalink.php?story_fbid=pfbid02XaWNiCVk69r1ghfvDVpujB8Hr9Y61uDvNakxiZFa2jwiPHscVdEQwCBHrmWZSyMRl&id=100051201402394).

This is a low-risk diff. There are **no run-time effects** and the diff has already been observed to compile locally. **If the code compiles, it works; test errors are spurious.**

#build_targets_regex[fbcode//glean/.*]

#buildmore - Be thorough

ig-no-test - No need to run instagram tests; build phase is sufficient.

#buildsonlynotests - **This diff has no runtime effects.**

 - If you approve of this diff, please use the "Accept & Ship" button :-)

(1 file modified.)

Reviewed By: palmje

Differential Revision: D67329001

fbshipit-source-id: 7ca569e2f330b195597cb0191b9266f01d41779a

glean/rocksdb/database-impl.cpp

@@ -269,7 +269,7 @@ struct SeekIterator final : rts::FactIterator {
   Fact::Ref get(Demand demand) override {
     if (iter_->Valid()) {
       auto key = input(iter_->key());
-      auto ty = key.fixed<Pid>();
+      [[maybe_unused]] auto ty = key.fixed<Pid>();
       assert(ty == type_);
       auto value = input(iter_->value());
       auto id = value.fixed<Id>();
@@ -278,7 +278,7 @@ struct SeekIterator final : rts::FactIterator {
       if (demand == KeyOnly) {
         return Fact::Ref{id, type_, Fact::Clause::fromKey(key.bytes())};
       } else {
-        auto found = db_->lookupById(id, slice_);
+        [[maybe_unused]] auto found = db_->lookupById(id, slice_);
         assert(found);
         return decomposeFact(id, slice_);
       }

glean/rts/stacked.h

@@ -160,11 +160,12 @@ struct Stacked<Define> final : StackedBase<Define> {
         //       similar). This means that there is no need to check against the
         //       value stored in the database as that must be empty, too.
         if (clause.value_size != 0) {
-          bool found = base->factById(id, [&](auto, auto found) {
-            if (clause.value() != found.value()) {
-              id = Id::invalid();
-            }
-          });
+          [[maybe_unused]] bool found =
+              base->factById(id, [&](auto, auto found) {
+                if (clause.value() != found.value()) {
+                  id = Id::invalid();
+                }
+              });
           assert(found);
         }
         return id;