scripted-diff: replace chainActive -> ::ChainActive()

Though at the moment ChainActive() simply references `g_chainstate.m_chain`,
doing this change now clears the way for multiple chainstate usage and allows
us to script the diff.

-BEGIN VERIFY SCRIPT-
git grep -l "chainActive" | grep -E '(h|cpp)$' | xargs sed -i '/chainActive =/b; /extern CChain& chainActive/b; s/\(::\)\{0,1\}chainActive/::ChainActive()/g'
-END VERIFY SCRIPT-

src/bench/bench.cpp

@@ -114,8 +114,8 @@ void benchmark::BenchRunner::RunAll(Printer& printer, uint64_t num_evals, double
     for (const auto& p : benchmarks()) {
         TestingSetup test{CBaseChainParams::REGTEST};
         {
-            assert(::chainActive.Height() == 0);
-            const bool witness_enabled{IsWitnessEnabled(::chainActive.Tip(), Params().GetConsensus())};
+            assert(::ChainActive().Height() == 0);
+            const bool witness_enabled{IsWitnessEnabled(::ChainActive().Tip(), Params().GetConsensus())};
             assert(witness_enabled);
         }