Fixed duplicate generation of forward declared class.

Fixes https://github.com/mono/CppSharp/issues/968.

src/Generator/Passes/ResolveIncompleteDeclsPass.cs

@@ -78,13 +78,13 @@ namespace CppSharp.Passes
             if (declaration.CompleteDeclaration != null)
                 return;
 
+            declaration.CompleteDeclaration =
+                ASTContext.FindCompleteClass(declaration.QualifiedName);
+
             var @class = declaration as Class;
             if (CheckForDuplicateForwardClass(@class))
                 return;
 
-            declaration.CompleteDeclaration =
-                ASTContext.FindCompleteClass(declaration.QualifiedName);
-
             if (declaration.CompleteDeclaration == null)
             {
                 declaration.GenerationKind = GenerationKind.Internal;

tests/CSharp/CSharp.h

@@ -1251,3 +1251,9 @@ public:
     virtual int virtualTakeArrays(Foo* fixedArrayOfPointersToObjects[3], int fixedArrayOfPrimitives[4],
                                   int* fixedArrayOfPointersToPrimitives[5]) const;
 };
+
+
+class TestForwardedClassInAnotherUnit
+{
+};
+

tests/CSharp/CSharpTemplates.h

@@ -472,3 +472,5 @@ template class DLL_API DerivedChangesTypeName<T1>;
 template class DLL_API TemplateWithIndexer<int>;
 template class DLL_API TemplateWithIndexer<T1>;
 template class DLL_API TemplateWithIndexer<T2*>;
+
+class TestForwardedClassInAnotherUnit;