Ruby: Attempt to fix performance of AppCandidate

`DataFlow::MethodNode.getAReturningNode` is expensive to compute.
Instead we look for rack responses which flow to the `SynthReturnNode`.
Each method has only one of these (vs many "returning" nodes) so it is
a lot faster.
I'm not sure yet whether the results are the same.

ruby/ql/lib/codeql/ruby/frameworks/Rack.qll

@@ -20,7 +20,7 @@ module Rack {
     AppCandidate() {
       call = this.getInstanceMethod("call") and
       call.getNumberOfParameters() = 1 and
-      isRackResponse(call.getAReturningNode())
+      exists(DataFlow::LocalSourceNode resp | isRackResponse(resp) | resp.flowsTo(call.getReturn()))
     }
 
     /**