validation: mark blocks building on an invalid block as BLOCK_FAILED_CHILD

Without doing so, header-only chains building on a chain that
will be marked as invalid would still be eligible for m_best_header.
This improves both getblockchaininfo and getchaintips behavior.

While this adds an iteration over the entire block index, it can only be
triggered by the user (invalidateblock) or by others at a cost (the
header needs to be accepted in the first place, so it needs valid PoW).

Co-authored-by: TheCharlatan <seb.kung@gmail.com>

src/validation.h

@@ -735,6 +735,9 @@ public:
         EXCLUSIVE_LOCKS_REQUIRED(!m_chainstate_mutex)
         LOCKS_EXCLUDED(::cs_main);
 
+    /** Set invalidity status to all descendants of a block */
+    void SetBlockFailureFlags(CBlockIndex* pindex) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+
     /** Remove invalidity status from a block and its descendants. */
     void ResetBlockFailureFlags(CBlockIndex* pindex) EXCLUSIVE_LOCKS_REQUIRED(cs_main);