Generate valid C# when a method from a secondary base has no native symbol

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

src/Generator/Generators/CSharp/CSharpSources.cs

@@ -629,7 +629,7 @@ namespace CppSharp.Generators.CSharp
 
             foreach (var method in @class.Methods)
             {
-                if (ASTUtils.CheckIgnoreMethod(method))
+                if (!method.IsGenerated || ASTUtils.CheckIgnoreMethod(method))
                     continue;
 
                 if (method.IsConstructor)
@@ -2755,6 +2755,13 @@ namespace CppSharp.Generators.CSharp
                 return;
             }
 
+            // ignored functions may get here from interfaces for secondary bases
+            if (function.Ignore)
+            {
+                WriteLine("throw new System.MissingMethodException(\"No C++ symbol to call.\");");
+                return;
+            }
+
             var retType = function.OriginalReturnType;
             if (returnType.Type == null)
                 returnType = retType;