[CSStep] Overload pruning should skip previously disabled choices

This should be a no-op for operators today expect that we found
a case were C++ imported operator that had a label for the second
argument which disabled the overload choice early and attempted
to disable it again here which breaks CSTrail because pruned
choices are re-enabled once disjunction checking is complete.

Author: xedin

lib/Sema/CSStep.h

@@ -708,6 +708,9 @@ class DisjunctionStep final : public BindingStep<DisjunctionChoiceProducer> {
       // Disable all of the overload choices which are different from
       // the one which is currently picked for representative.
       for (auto *constraint : disjunction->getNestedConstraints()) {
+        if (constraint->isDisabled())
+          continue;
+
         auto choice = constraint->getOverloadChoice();
         if (!choice.isDecl() || choice.getDecl() == representative.getDecl())
           continue;