[CSStep] Abort any binding step when constraint system is 'too complex'

Previously the "too complex" would be detected by `ComponentStep::take`
but binding steps would still proceed until the producer is exhausted
because it considers failure of the previous resume to be just a failed
choice.

Author: xedin

lib/Sema/CSStep.h

@@ -504,6 +504,11 @@ template <typename P> class BindingStep : public SolverStep {
 
 public:
   StepResult take(bool prevFailed) override {
+    // Before attempting the next choice, let's check whether the constraint
+    // system is too complex already.
+    if (CS.getExpressionTooComplex(Solutions))
+      return done(/*isSuccess=*/false);
+
     while (auto choice = Producer()) {
       if (shouldSkip(*choice))
         continue;