[CSSolver] Attempt choices marked as disfavored at the end of partition

Since such choices are all but guaranteed to be worst than any
other non-disfavored choice, let's attempt them last to avoid
having to form a complete solution just to filter it out during
ranking.

Author: xedin

lib/Sema/CSSolver.cpp

@@ -2310,6 +2310,8 @@ void DisjunctionChoiceProducer::partitionDisjunction(
   // end of the partitioning.
   SmallVector<unsigned, 4> favored;
   SmallVector<unsigned, 4> everythingElse;
+  // Disfavored choices are part of `everythingElse` but introduced at the end.
+  SmallVector<unsigned, 4> disfavored;
   SmallVector<unsigned, 4> simdOperators;
   SmallVector<unsigned, 4> disabled;
   SmallVector<unsigned, 4> unavailable;
@@ -2342,6 +2344,11 @@ void DisjunctionChoiceProducer::partitionDisjunction(
         everythingElse.push_back(index);
         return true;
       }
+
+      if (decl->getAttrs().hasAttribute<DisfavoredOverloadAttr>()) {
+        disfavored.push_back(index);
+        return true;
+      }
     }
 
     return false;
@@ -2385,6 +2392,9 @@ void DisjunctionChoiceProducer::partitionDisjunction(
     return true;
   });
 
+  // Introduce disfavored choices at the end.
+  everythingElse.append(disfavored);
+
   // Local function to create the next partition based on the options
   // passed in.
   PartitionAppendCallback appendPartition =