[CS] NFC: Make failedConstraint private

Author: hamishknight

lib/Sema/CSStep.cpp

@@ -260,7 +260,7 @@ StepResult DependentComponentSplitterStep::take(bool prevFailed) {
   // "split" is considered a failure if previous step failed,
   // or there is a failure recorded by constraint system, or
   // system can't be simplified.
-  if (prevFailed || CS.failedConstraint || CS.simplify())
+  if (prevFailed || CS.getFailedConstraint() || CS.simplify())
     return done(/*isSuccess=*/false);
 
   // Figure out the sets of partial solutions that this component depends on.

lib/Sema/ConstraintSystem.h

@@ -1613,8 +1613,6 @@ class ConstraintSystem {
 
   class SolverScope;
 
-  Constraint *failedConstraint = nullptr;
-
   /// Expressions that are known to be unevaluated.
   /// Note: this is only used to support ObjCSelectorExpr at the moment.
   llvm::SmallPtrSet<Expr *, 2> UnevaluatedRootExprs;
@@ -1623,6 +1621,10 @@ class ConstraintSystem {
   unsigned CountDisjunctions = 0;
 
 private:
+  /// A constraint that has failed along the current solver path.
+  /// Do not set directly, call \c recordFailedConstraint instead.
+  Constraint *failedConstraint = nullptr;
+
   /// Current phase of the constraint system lifetime.
   ConstraintSystemPhase Phase = ConstraintSystemPhase::ConstraintGeneration;
 
@@ -2098,6 +2100,10 @@ class ConstraintSystem {
   };
 
 public:
+  /// Retrieve the first constraint that has failed along the solver's path, or
+  /// \c nullptr if no constraint has failed.
+  Constraint *getFailedConstraint() const { return failedConstraint; }
+
   ConstraintSystemPhase getPhase() const { return Phase; }
 
   /// Move constraint system to a new phase of its lifetime.