Editorial: Fix type mismatch for class without constructor

[mobius29]

When a class has no explicit constructor, ClassDefinitionEvaluation step 14.b creates the default constructor via CreateBuiltinFunction.
Later, step 14.a.vi calls InitializeInstanceElements(result, F), which expects its second parameter constructor to be an ECMAScript Function Object. However, actual F's type is a Built-in Function Object and this causes type mismatch.

For example:

class A {}
var x = new A();

This PR fixes the inconsistency in two parts

• Add built-in function object type to InitializeInstanceElements' second parameter(constructor)
  In step 16, MakeConstructor already allows a Built-in Function Object type, so this brings the earlier step into line.
• Add [[PrivateMethods]] and [[Fields]] to additionalInternalSlotList when calling CreateBuiltinFunction in step 14.b
  Without these slots a built-in constructor produced for a class would not have the properties that InitializeInstanceElements step 1 and 3 and ClassDefinitionEvaluation steps 27 and 28 immediately access, causing an invalid operation (see Synthesized constructor in ClassDefinitionEvaluation is missing [[PrivateMethods]] and [[Fields]] #3204).

Fixes #3204.

[michaelficarra]

LGTM. Steps 28 and 29 were already attempting to initialise [[PrivateMethods]] and [[Fields]], so this looks complete.

[linusg]

LGTM, thanks!