Don't display alias warnings for self-defined identifiers

[Vannevelj]

When you have an enum like this:

public enum TypeCode
{
        Object = 1,
        DBNull = 2,
        Boolean = 3,
        Char = 4,
        SByte = 5,
        Byte = 6,
}

it will incorrectly suggest you to change the usages to aliases. For example this now displays a warning:

case TypeCode.Boolean:

I imagine this is the same for any self-defined type.

[Vannevelj]

This also presents itself here:

return _tree.Types.Single(t => t.Name == name);

Thinking this is the type Single instead. We can probably just check GetTypeInfo().SpecialType.