Add synthetic constructors to class member scope, so they won't fly 'in the air' in the backends

This is required mainly for Native compiler since it wont't work correctly on descriptors that are not present in the class.

compiler/backend/src/org/jetbrains/kotlin/codegen/ConstructorCodegen.java

@@ -130,6 +130,8 @@ public class ConstructorCodegen {
         ConstructorContext constructorContext = context.intoConstructor(constructorDescriptor, typeMapper);
 
         KtSecondaryConstructor constructor = (KtSecondaryConstructor) descriptorToDeclaration(constructorDescriptor);
+        // Synthetic constructors don't have corresponding declarations
+        if (constructor == null) return;
 
         functionCodegen.generateMethod(
                 JvmDeclarationOriginKt.OtherOrigin(constructor, constructorDescriptor),