[CSSolver] Add skeleton of iterative solve

The idea so to split solving into non-recursive steps,
represented by `SolverStep`, each of the steps is resposible
for a unit of work e.g. attempting type variable or
disjunction bindings/choices.

Each step could produce more work via "follow-up" steps,
complete "partial" solution when it's done, or error which
terminates solver loop.

lib/Sema/ConstraintSystem.h

@@ -57,6 +57,7 @@ class DisjunctionChoiceProducer;
 class TypeBinding;
 class TypeVariableBinding;
 class TypeVarBindingProducer;
+class SolverStep;
 
 } // end namespace constraints
 
@@ -930,6 +931,7 @@ public:
   friend class FailureDiagnostic;
   friend class TypeVarBindingProducer;
   friend class TypeVariableBinding;
+  friend class SolverStep;
 
   class SolverScope;
 
@@ -3088,6 +3090,9 @@ public:
   /// \returns true if there are no solutions
   bool solveRec(SmallVectorImpl<Solution> &solutions);
 
+  bool solveIteratively(Expr *expr, SmallVectorImpl<Solution> &solutions,
+                        FreeTypeVariableBinding allowFreeTypeVariables);
+
   /// \brief Solve the system of constraints.
   ///
   /// \param allowFreeTypeVariables How to bind free type variables in