[ConstraintSystem] Remove unnecessary forward declarations of the steps

Also move `DisjunctionStep` from being a friend of `ConstraintSystem`
because it's not strictly necessary.

Author: xedin

lib/Sema/CSStep.cpp

@@ -612,7 +612,7 @@ bool DisjunctionStep::attemptChoice(const DisjunctionChoice &choice) {
   // took for it.
   if (auto *disjunctionLocator = Producer.getLocator()) {
     auto index = choice.getIndex();
-    CS.DisjunctionChoices.push_back({disjunctionLocator, index});
+    recordDisjunctionChoice(disjunctionLocator, index);
 
     // Implicit unwraps of optionals are worse solutions than those
     // not involving implicit unwraps.

lib/Sema/CSStep.h

@@ -32,6 +32,7 @@ namespace swift {
 namespace constraints {
 
 class SolverStep;
+class ComponentStep;
 
 /// Represents available states which every
 /// given step could be in during it's lifetime.
@@ -200,6 +201,11 @@ class SolverStep {
     return CS.resolvedOverloadSets;
   }
 
+  void recordDisjunctionChoice(ConstraintLocator *disjunctionLocator,
+                               unsigned index) const {
+    CS.DisjunctionChoices.push_back({disjunctionLocator, index});
+  }
+
   Score getCurrentScore() const { return CS.CurrentScore; }
 
   Optional<Score> getBestScore() const { return CS.solverState->BestScore; }

lib/Sema/ConstraintSystem.h

@@ -53,16 +53,6 @@ namespace constraints {
 class ConstraintGraph;
 class ConstraintGraphNode;
 class ConstraintSystem;
-class DisjunctionChoiceProducer;
-class TypeBinding;
-class TypeVariableBinding;
-class TypeVarBindingProducer;
-class StepScope;
-class SolverStep;
-class SplitterStep;
-class ComponentStep;
-class TypeVariableStep;
-class DisjunctionStep;
 
 } // end namespace constraints
 
@@ -941,7 +931,6 @@ class ConstraintSystem {
   friend class SplitterStep;
   friend class ComponentStep;
   friend class TypeVariableStep;
-  friend class DisjunctionStep;
 
   class SolverScope;