Generic attributes should disallow type arguments which are disallowed by 'typeof'

[RikkiGibson]

Version Used: 3661e83

Steps to Reproduce:

using System;

class Attr1 : Attribute
{
    public Attr1(Type t) { }
}

class Attr2<T> : Attribute { }

[Attr1(typeof(dynamic))]
[Attr2<dynamic>]
class C { }

Expected Behavior: Error on typeof(dynamic) as well as on Attr2<dynamic>.

Actual Behavior: Error only on typeof(dynamic).

We want to do this because when we emit attribute type arguments there's nowhere to put things like DynamicAttribute, for example. So the fact that the type argument is dynamic cannot round trip.

LDM has tentatively expressed support for the change. We should still probably create a speclet in csharplang for this feature so that all these behaviors are documented.