SSA/Ruby: Address review comments.

ruby/ql/lib/codeql/ruby/dataflow/internal/SsaImpl.qll

@@ -479,6 +479,10 @@ private module DataFlowIntegrationInput implements Impl::DataFlowIntegrationInpu
 
   Expr getARead(Definition def) { result = Cached::getARead(def) }
 
+  predicate ssaDefHasSource(WriteDefinition def) {
+    any(ParameterExt p).isInitializedBy(def) or def.(Ssa::WriteDefinition).assigns(_)
+  }
+
   class Guard extends Cfg::CfgNodes::AstCfgNode {
     /**
      * Holds if the control flow branching from `bb1` is dependent on this guard,

shared/ssa/codeql/ssa/Ssa.qll

@@ -1690,7 +1690,7 @@ module Make<LocationSig Location, InputSig<Location> Input> {
       /** Gets the underlying definition. */
       WriteDefinition getDefinition() { result = def }
 
-      override string toString() { result = def.toString() }
+      override string toString() { result = "[source] " + def.toString() }
 
       override Location getLocation() { result = def.getLocation() }
     }