[CSStep] Conjunction: Tranform all unbound outer variables into placeholders during ambiguity

Since conjunction is not going to continue current solver path
after discovering an ambiguity, let's just mark all of the
unbound outer variables as placeholders to produce a complete solution.

(cherry picked from commit 609b3e5)

Author: xedin

lib/Sema/CSStep.cpp

@@ -960,6 +960,16 @@ StepResult ConjunctionStep::resume(bool prevFailed) {
           // element does contribute to the overall score.
           restoreOriginalScores();
 
+          // Transform all of the unbound outer variables into
+          // placeholders since we are not going to solve for
+          // each ambguous solution.
+          for (auto *typeVar : CS.getTypeVariables()) {
+            if (!typeVar->getImpl().hasRepresentativeOrFixed()) {
+              CS.assignFixedType(
+                  typeVar, PlaceholderType::get(CS.getASTContext(), typeVar));
+            }
+          }
+
           // Note that `worseThanBestSolution` isn't checked
           // here because `Solutions` were pre-filtered, and
           // outer score is the same for all of them.