More global:: scenarios

[333fred]

The fix for #43960 didn't cover all scenarios this can occur, and the VS build is still failing. An example:

namespace N
{
    class C
    {
        void M(object o)
        {
            if (o is global::N.C) { }
        }
    }
}

[gafter]

Here is a repro:

        [Fact, WorkItem(44019, "https://github.com/dotnet/roslyn/issues/44019")]
        public void NamespaceQualifiedEnumConstantInIsPattern_03()
        {
            var source =
@"namespace E
{
    public class A { }
}

class Class1
{
    void M(object e)
    {
        if (e is global::E.A) { }
    }
}";
            CreateCompilation(source, parseOptions: TestOptions.Regular7, options: TestOptions.ReleaseDll).VerifyDiagnostics(
                );
            CreatePatternCompilation(source, options: TestOptions.ReleaseDll).VerifyDiagnostics(
                );
        }