In C# binder, suggest using UnknownResultType instead of CreateErrorType()

[gafter]

In the C# binders, we currently use CreateErrorType() to create an error type during error recovery. However, we have a cached singleton error type we could use instead called ErrorTypeSymbol.UnknownResultType. Consider refactoring the former to the latter throughout the compiler.

(Thanks to @AlekseyTs for observing this)

[gafter]

The reason not to do this is that UnknownResultType is not merely an error type. Its identity is used throughout the compilers as a flag to indicate that a type stored in an internal variable has not been computed yet. For example

        /// <summary>
        /// For enum types, gets the underlying type. Returns null on all other
        /// kinds of types.
        /// </summary>
        public override NamedTypeSymbol EnumUnderlyingType
        {
            get
            {
                if (ReferenceEquals(_lazyEnumUnderlyingType, ErrorTypeSymbol.UnknownResultType))
                {
                    DiagnosticBag diagnostics = DiagnosticBag.GetInstance();
                    if ((object)Interlocked.CompareExchange(ref _lazyEnumUnderlyingType, this.GetEnumUnderlyingType(diagnostics), ErrorTypeSymbol.UnknownResultType) ==
                        (object)ErrorTypeSymbol.UnknownResultType)
                    {
                        AddDeclarationDiagnostics(diagnostics);
                        this.state.NotePartComplete(CompletionPart.EnumUnderlyingType);
                    }
                    diagnostics.Free();
                }

                return _lazyEnumUnderlyingType;
            }
        }