VB -> C#: Improve date literal conversion

[GrahamTheCoder]

In #159 I said

This will need to deal with inlining a const field as well as just constant literals.

I didn't actually implement this part so it will now leave you with a compile time error. Something like:

Input:

Public Class Test
    Const x As Date = #1990-1-1#
    Private Sub Y(ByVal opt As Date = x)
    End Sub
End Class

Erroneous output:

using System;

public class Test
{
    DateTime x = DateTime.Parse("1990-01-01");
    private void Y(DateTime opt = x)
    {
    }
}

Expected output:

using System;

public class Test
{
    private static DateTime x = DateTime.Parse("1990-01-01");
    private void Y([Optional, DateTimeConstant(123456789L/*1990-01-01*/)] DateTime opt)
    {
    }
}

Proposed work:

• Inline the tick count when a const field is used as an optional parameter
• Add a comment for what date it represents in the existing case and this case
• Make const date fields into static fields