Switch on enum type nested in generic type produces LangVersion error

[RikkiGibson]

I think this is related to #34678.

Version Used:
master as of April 9 (commit 43f2329)

Steps to Reproduce:

1. Create a project using non-preview LangVersion
2. Write the following code

public class C<T>
{
    public enum E
    {
        V1, V2
    }

    public void M()
    {
        switch(default(E))
        {
            case E.V1: // error CS8511
            break;
        }
    }
}

Expected Behavior:
Builds successfully (as it does in dev15.9)

Actual Behavior:

CS8511	An expression of type 'C<T>.E' cannot be handled by a pattern of type 'C<T>.E'. Please use language version 'preview' or greater to match an open type with a constant pattern.