Merge #13480: Avoid copies in range-for loops and add a warning to detect them

d92204c900 build: add warning to detect hidden copies in range-for loops (Cory Fields)
466e16e0e8 cleanup: avoid hidden copies in range-for loops (Cory Fields)

Pull request description:

  Following-up on #13241, which was itself a follow-up of #12169.

  See title. Fixing these would otherwise be a continuous process, adding the warning should keep them from cropping up.

  Note that the warning seems to be Clang-only for now.

Tree-SHA512: ccfb769c3128b3f92c95715abcf21ee2496fe2aa384f80efead1529a28eeb56b98995b531b49a089f8142601389e63f7bb935963d724eacde4f5e1b4a024934b

src/validation.cpp

@@ -651,7 +651,7 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
         view.SetBackend(viewMemPool);
 
         // do all inputs exist?
-        for (const CTxIn txin : tx.vin) {
+        for (const CTxIn& txin : tx.vin) {
             if (!pcoinsTip->HaveCoinInCache(txin.prevout)) {
                 coins_to_uncache.push_back(txin.prevout);
             }
@@ -957,7 +957,7 @@ static bool AcceptToMemoryPoolWorker(const CChainParams& chainparams, CTxMemPool
         }
 
         // Remove conflicting transactions from the mempool
-        for (const CTxMemPool::txiter it : allConflicting)
+        for (CTxMemPool::txiter it : allConflicting)
         {
             LogPrint(BCLog::MEMPOOL, "replacing tx %s with %s for %s BTC additional fees, %d delta bytes\n",
                     it->GetTx().GetHash().ToString(),