Add clarifying comment to DependencyResolver.

* Adds a clarifying comment to explain a surprising implementation.

PiperOrigin-RevId: 493694784
Change-Id: I37cbf23153761a3539acd6fe47deba5a3bdd01a0

src/main/java/com/google/devtools/build/lib/analysis/DependencyResolver.java

@@ -751,6 +751,9 @@ private void addAspectAttributes(
   private static AspectCollection computeAspectCollections(
       ImmutableList<Aspect> aspects, Target toTarget) throws InconsistentAspectOrderException {
     if (toTarget instanceof OutputFile) {
+      // When applyToGeneratingRules holds, the aspect cannot have required providers so it's
+      // possible to skip the filtering that happens further below. However,
+      // apply_to_generating_rules is rare in the codebase so the optimization is not worth it.
       aspects =
           aspects.stream()
               .filter(aspect -> aspect.getDefinition().applyToGeneratingRules())