roslyn: [Bug] Interpolated strings in attributes

Version Used: CS 6

Steps to Reproduce:

    class SomeClass
    {
        public const String FirstNameDef = "Pavel";

        [Required(ErrorMessage = "Field 'FirstName' is required.")]

        [Required(ErrorMessage = $"Field {nameof(FirstName)} is required.")] 
        // Error: An attribute argument must be a constant expression, 
        // typeof expression or array creation expression of an attribute parameter type

        [DefaultValue(FirstNameDef)]

        [DefaultValue($"{FirstNameDef}")]  
        // Error: An attribute argument must be a constant expression, 
        // typeof expression or array creation expression of an attribute parameter type

        public String FirstName {
            get { return _firstName; }
            set { _firstName = value; }
        }

        public String _firstName = $"{FirstNameDef}";
    }