Revert "[CSGen] Remove ConstraintOptimizer and all favoring logic"

This reverts commit 4432c51.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -5436,6 +5436,11 @@ class ConstraintSystem {
   bool applySolutionToBody(TapExpr *tapExpr,
                            SyntacticElementTargetRewriter &rewriter);
 
+  /// Reorder the disjunctive clauses for a given expression to
+  /// increase the likelihood that a favored constraint will be successfully
+  /// resolved before any others.
+  void optimizeConstraints(Expr *e);
+
   void startExpressionTimer(ExpressionTimer::AnchorType anchor);
 
   /// Determine if we've already explored too many paths in an