It is a error in VB when I declare a Constant of type Some Class = Nothing

Version Used:
15.6.4
Steps to Reproduce:
```
Imports System
Namespace Test
Friend MustInherit Partial Class ConstantValue
Implements IEquatable(Of ConstantValue)
Public Const NotAvailable As ConstantValue = Nothing
End Class
End Namespace
```
**Expected Behavior**:
No errors when setting NotAvailable to Nothing. This works in C# and the IL allows it. or if I declare in a Module NotAvailable ReadOnly. I would also expect this to work for Structure, and other places it works in C# and is allowed in the IL.
**Actual Behavior**:
BC30424 Constants must be of an intrinsic or enumerated type, not a class, structure, type parameter, or array type.

Reproduce the VB snippet from the issue and inspect how the compiler handles the BC30424 diagnostic for constant declarations. Compare the accepted C# and IL behavior described in the issue, then determine the language-design and compatibility requirements before changing the rule; done means the sample and the related class, structure, and other cases behave as specified.

Written by the indexing model from the issue text.