CLS compliance assert fails on extension blocks

[jjonescz]

Version Used: de9d72e

Steps to Reproduce:

[Fact]
public void Cls()
{
    var src = """
        [assembly: System.CLSCompliant(true)]
        public static class Extensions
        {
            extension(object) { }
        }
        """;
    CreateCompilation(src).VerifyEmitDiagnostics();
}

Fails on this assert because the extension block has no base:

roslyn/src/Compilers/CSharp/Portable/Compiler/ClsComplianceChecker.cs

Line 532 in e3cf188

System.Diagnostics.Debug.Assert((object)baseType != null || symbol.SpecialType == SpecialType.System_Object, "Only object has no base.");