Revert back visibility in deserialization ctor to public so inheritors can access it from other modules.

Fix instantiating of polymorphic serializer with generics.
Move context creation below as asked.

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

@@ -127,12 +127,12 @@ public class ConstructorCodegen {
     ) {
         if (!canHaveDeclaredConstructors(descriptor)) return;
 
-        ConstructorContext constructorContext = context.intoConstructor(constructorDescriptor, typeMapper);
-
         KtSecondaryConstructor constructor = (KtSecondaryConstructor) descriptorToDeclaration(constructorDescriptor);
         // Synthetic constructors don't have corresponding declarations
         if (constructor == null) return;
 
+        ConstructorContext constructorContext = context.intoConstructor(constructorDescriptor, typeMapper);
+
         functionCodegen.generateMethod(
                 JvmDeclarationOriginKt.OtherOrigin(constructor, constructorDescriptor),
                 constructorDescriptor, constructorContext,