Add flowCheckNodeSpecific

This allows individual languages to specify `FlowCheckNode`s, which
break up the big step relation and make sure that those nodes appear in
path summaries.

Author: owen-mc

go/ql/lib/semmle/go/dataflow/internal/DataFlowImpl.qll

@@ -2021,7 +2021,8 @@ module Impl<FullStateConfigSig Config> {
       FlowCheckNode() {
         castNode(this.asNode()) or
         clearsContentCached(this.asNode(), _) or
-        expectsContentCached(this.asNode(), _)
+        expectsContentCached(this.asNode(), _) or
+        flowCheckNodeSpecific(this.asNode())
       }
     }