Duplicate RazorDiagnostic IDs for two different errors

[Ajay Bhargav B]

https://github.com/aspnet/Razor/blob/master/src/Microsoft.AspNetCore.Razor.Language/RazorDiagnosticFactory.cs#L235 and https://github.com/aspnet/Razor/blob/master/src/Microsoft.AspNetCore.Razor.Language/RazorDiagnosticFactory.cs#L245 both have the same diagnostic id RZ1020. We need to fix it.

@rynowak is this a breaking change?

[Ryan Nowak]

I think we're free to change this.

[Artak]

@ajaybhargavb, all yours!

[Ajay Bhargav B]

8674dd9