[ConstraintGraph] Make Changes member types public and create accessor for Active Scope starting index for use in printing.

Author: Amritpan Kaur

include/swift/Sema/ConstraintGraph.h

@@ -458,6 +458,7 @@ class ConstraintGraph {
   /// Each change can be undone (once, and in reverse order) by calling the
   /// undo() method.
   class Change {
+  public:
     /// The kind of change.
     ChangeKind Kind;
 
@@ -482,7 +483,6 @@ class ConstraintGraph {
       } Binding;
     };
 
-  public:
     Change() : Kind(ChangeKind::AddedTypeVariable), TypeVar(nullptr) { }
 
     /// Create a change that added a type variable.

include/swift/Sema/ConstraintGraphScope.h

@@ -50,6 +50,11 @@ class ConstraintGraphScope {
 public:
   explicit ConstraintGraphScope(ConstraintGraph &CG);
   ~ConstraintGraphScope();
+  
+  /// Get number of changes recorded at the start of the current active scope.
+  unsigned getStartIdx() {
+    return NumChanges;
+  }
 };
 
 } // end namespace constraints