Improve error message for explicit cast of collection literal to non-generic type

[cston]

Compile:

using System;
using System.Collections;
using System.Collections.Generic;

class C : IEnumerable<int>
{
    public void Add(int i) { }
    public IEnumerator<int> GetEnumerator() => null;
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

class Program
{
    static void Main()
    {
        var c = (C)[1];
    }
}

Actual Behavior:

(16,18): error CS0119: 'C' is a type, which is not valid in the given context

Expected Behavior:
Diagnostic above, with suggestion to parenthesize the collection literal if a cast is intended.

See #68793 (comment)

Relates to test plan #66418

[RikkiGibson]

I thought we had made a decision that we would break this scenario and actually cause it to mean "cast collection literal [1] to type C", with the expectation that real-world code which intends for this to be an indexer access is overwhelmingly likely to be written as C[1] and not (C)[1].

@CyrusNajmabadi am I remembering this wrong?

[CyrusNajmabadi]

I am not remembering any decision to make a breaking change here. That would be a fairly big deal, and we definitely weren't making breaking change decisions later in 12.

We might want to consider the breaking change in hte future though, esp if we have a breaking change mechanism finally decided upon.