Ref safety diagnostics downgraded to warnings even in some safe contexts

[jjonescz]

Version Used: 5.7.0-1.26217.5 (77dc97b)

Steps to Reproduce:

.NET Lab

using System.Collections.Generic;
using System.Runtime.CompilerServices;

class C
{
    unsafe void M1()
    {
        int p = *default(int*); // no error
        ref int outside = ref Unsafe.NullRef<int>();
        {
            int local = 1;
            outside = ref local; // warning
        }
    }

    unsafe IEnumerable<int> M2()
    {
        int p = *default(int*); // error
        ref int outside = ref Unsafe.NullRef<int>();
        {
            int local = 1;
            outside = ref local; // warning
        }
        yield return 1;
    }
}

Expected Behavior: the ref safety violation in M2 should be an error, since it's in a safe context

Actual Behavior: it's a warning (it thinks it's in unsafe context since M2 is declared as unsafe but doesn't consider that it's an iterator)