refactor: Enable clang-tidy bugprone-unused-return-value

This requires some small refactors to silence false-positive warnings.

Also, expand the bugprone-unused-return-value.CheckedReturnTypes option
to include util::Result, and util::Expected.

src/bench/coin_selection.cpp

@@ -130,7 +130,7 @@ static void BnBExhaustion(benchmark::Bench& bench)
     bench.run([&] {
         // Benchmark
         CAmount target = make_hard_case(17, utxo_pool);
-        SelectCoinsBnB(utxo_pool, target, 0, MAX_STANDARD_TX_WEIGHT); // Should exhaust
+        [[maybe_unused]] auto _{SelectCoinsBnB(utxo_pool, target, /*cost_of_change=*/0, MAX_STANDARD_TX_WEIGHT)}; // Should exhaust
 
         // Cleanup
         utxo_pool.clear();