Marked the __Instance property in interfaces as synthetised.

Signed-off-by: Dimitar Dobrev <dpldobrev@yahoo.com>

src/AST/Function.cs

@@ -93,7 +93,8 @@ namespace CppSharp.AST
         None,
         ComplementOperator,
         AbstractImplCall,
-        DefaultValueOverload
+        DefaultValueOverload,
+        InterfaceInstance
     }
 
     public class Function : Declaration, ITypedDecl, IMangledDecl

src/AST/Property.cs

@@ -115,6 +115,11 @@ namespace CppSharp.AST
             }
         }
 
+        public bool IsSynthetized
+        {
+            get { return GetMethod != null && GetMethod.IsSynthetized; }
+        }
+
         public override T Visit<T>(IDeclVisitor<T> visitor)
         {
             return visitor.VisitProperty(this);

src/Generator/Passes/MultipleInheritancePass.cs

@@ -84,7 +84,11 @@ namespace CppSharp.Passes
                 instance.Namespace = @interface;
                 instance.Name = Helpers.InstanceIdentifier;
                 instance.QualifiedType = new QualifiedType(new BuiltinType(PrimitiveType.IntPtr));
-                instance.GetMethod = new Method { Namespace = @interface };
+                instance.GetMethod = new Method
+                {
+                    SynthKind = FunctionSynthKind.InterfaceInstance,
+                    Namespace = @interface
+                };
                 @interface.Properties.Add(instance);
             }