[validation] re-introduce bool for whether a transaction is RBF

This bool was originally part of Workspace and was removed in bitcoin#22539
when it was no longer needed in Finalize(). Re-introducing it because,
once again, multiple functions will need to know whether we're doing an
RBF. Member of MemPoolAccept so that we can use this to inform package
RBF in the future.

src/validation.cpp

@@ -568,6 +568,9 @@ class MemPoolAccept
     // in-mempool conflicts; see below).
     size_t m_limit_descendants;
     size_t m_limit_descendant_size;
+
+    /** Whether the transaction(s) would replace any mempool transactions. If so, RBF rules apply. */
+    bool m_rbf{false};
 };
 
 bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
@@ -808,8 +811,8 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         return state.Invalid(TxValidationResult::TX_CONSENSUS, "bad-txns-spends-conflicting-tx", *err_string);
     }
 
-
-    if (!setConflicts.empty()) {
+    m_rbf = !setConflicts.empty();
+    if (m_rbf) {
         CFeeRate newFeeRate(nModifiedFees, nSize);
         // It's possible that the replacement pays more fees than its direct conflicts but not more
         // than all conflicts (i.e. the direct conflicts have high-fee descendants). However, if the